Status | Authors | Coach | DRIs | Owning Stage | Created |
---|---|---|---|---|---|
ongoing |
@grzesiek
|
@ayufan
@grzesiek
|
@jreporter
@cheryl.li
| devops verify | 2022-05-31 |
Pipeline data partitioning design
What problem are we trying to solve?
We want to partition the CI/CD dataset, because some of the database tables are extremely large, which might be challenging in terms of scaling single node reads, even after we ship the CI/CD database decomposition.
We want to reduce the risk of database performance degradation by transforming a few of the largest database tables into smaller ones using PostgreSQL declarative partitioning.
See more details about this effort in the parent blueprint.
How are CI/CD data decomposition, partitioning, and time-decay related?
CI/CD decomposition is an extraction of a CI/CD database cluster out of the “main” database cluster, to make it possible to have a different primary database receiving writes. The main benefit is doubling the capacity for writes and data storage. The new database cluster will not have to serve reads / writes for non-CI/CD database tables, so this offers some additional capacity for reads too.
CI/CD partitioning is dividing large CI/CD database tables into smaller ones. This will improve reads capacity on every CI/CD database node, because it is much less expensive to read data from small tables, than from large multi-terabytes tables. We can add more CI/CD database replicas to better handle the increase in the number of SQL queries that are reading data, but we need partitioning to perform a single read more efficiently. Performance in other aspects will improve too, because PostgreSQL will be more efficient in maintaining multiple small tables than in maintaining a very large database table.
CI/CD time-decay allows us to benefit from the strong time-decay characteristics of pipeline data. It can be implemented in many different ways, but using partitioning to implement time-decay might be especially beneficial. When implementing a time decay we usually mark data as archived, and migrate it out of a database to a different place when data is no longer relevant or needed. Our dataset is extremely large (tens of terabytes), so moving such a high volume of data is challenging. When time-decay is implemented using partitioning, we can archive the entire partition (or set of partitions) by updating a single record in one of our database tables. It is one of the least expensive ways to implement time-decay patterns at a database level.
Why do we need to partition CI/CD data?
We need to partition CI/CD data because our database tables storing pipelines,
builds, and artifacts are too large. The ci_builds
database table size is
currently around 2.5 TB with an index of around 1.4 GB. This is too much and
violates our principle of 100 GB max size.
We also want to build alerting
to notify us when this number is exceeded.
Large SQL tables increase index maintenance time, during which freshly deleted tuples
cannot be cleaned by autovacuum
. This highlight the need for small tables.
We will measure how much bloat we accumulate when (re)indexing huge tables. Base on this analysis,
we will be able to set up SLO (dead tuples / bloat), associated with (re)indexing.
We’ve seen numerous S1 and S2 database-related production environment incidents, over the last couple of months, for example:
- S1: 2022-03-17 Increase in writes in
ci_builds
table - S1: 2021-11-22 Excessive buffer read in replicas for
ci_job_artifacts
- S2: 2022-04-12 Transactions detected that have been running for more than 10m
- S2: 2022-04-06 Database contention plausibly caused by excessive
ci_builds
reads - S2: 2022-03-18 Unable to remove a foreign key on
ci_builds
- S2: 2022-10-10 The
queuing_queries_duration
SLI apdex violating SLO
We have approximately 50 ci_*
prefixed database tables, and some of them
would benefit from partitioning.
A simple SQL query to get this data:
WITH tables AS (SELECT table_name FROM information_schema.tables WHERE table_name LIKE 'ci_%')
SELECT table_name,
pg_size_pretty(pg_total_relation_size(quote_ident(table_name))) AS total_size,
pg_size_pretty(pg_relation_size(quote_ident(table_name))) AS table_size,
pg_size_pretty(pg_indexes_size(quote_ident(table_name))) AS index_size,
pg_total_relation_size(quote_ident(table_name)) AS total_size_bytes
FROM tables ORDER BY total_size_bytes DESC;
See data from March 2022:
Table name | Total size | Index size |
---|---|---|
ci_builds
| 3.5 TB | 1 TB |
ci_builds_metadata
| 1.8 TB | 150 GB |
ci_job_artifacts
| 600 GB | 300 GB |
ci_pipelines
| 400 GB | 300 GB |
ci_stages
| 200 GB | 120 GB |
ci_pipeline_variables
| 100 GB | 20 GB |
(…around 40 more) |
Based on the table above, it is clear that there are tables with a lot of stored data.
While we have almost 50 CI/CD-related database tables, we are initially interested in partitioning only 6 of them. We can start by partitioning the most interesting tables in an iterative way, but we also should have a strategy for partitioning the remaining ones if needed. This document is an attempt to capture this strategy, describe as many details as possible, to share this knowledge among engineering teams.
How do we want to partition CI/CD data?
We want to partition the CI/CD tables in iterations. It might not be feasible to partition all of the 6 initial tables at once, so an iterative strategy might be necessary. We also want to have a strategy for partitioning the remaining database tables when it becomes necessary.
It is also important to avoid large data migrations. We store almost 6
terabytes of data in the biggest CI/CD tables, in many different columns and
indexes. Migrating this amount of data might be challenging and could cause
instability in the production environment. Due to this concern, we’ve developed
a way to attach an existing database table as a partition zero without downtime
and excessive database locking, what has been demonstrated in one of the
first proofs of concept.
This makes creation of a partitioned schema possible without a downtime (for
example using a routing table p_ci_pipelines
), by attaching an existing
ci_pipelines
table as partition zero without exclusive locking. It will be
possible to use the legacy table as usual, but we can create the next partition
when needed and the p_ci_pipelines
table will be used for routing queries. To
use the routing table we need to find a good partitioning key.
Our plan is to use logical partition IDs. We want to start with the
ci_pipelines
table and create a partition_id
column with a DEFAULT
value
of 100
or 1000
. Using a DEFAULT
value avoids the challenge of backfilling
this value for every row. Adding a CHECK
constraint prior to attaching the
first partition tells PostgreSQL that we’ve already ensured consistency and
there is no need to check it while holding an exclusive table lock when
attaching this table as a partition to the routing table (partitioned schema
definition). We will increment this value every time we create a new partition
for p_ci_pipelines
, and the partitioning strategy will be LIST
partitioning.
We will also create a partition_id
column in the other initial 6 database
tables we want to iteratively partition. After a new pipeline is created, it
will get a partition_id
assigned, and all the related resources, like builds
and artifacts, will share the same value. We want to add the partition_id
column into all 6 problematic tables because we can avoid backfilling this data
when we decide it is time to start partitioning them.
We want to partition CI/CD data iteratively. We plan to start with the
ci_builds_metadata
table, because this is the fastest growing table in the CI
database and want to contain this rapid growth. This table has also the most
simple access patterns - a row from it is being read when a build is exposed to
a runner, and other access patterns are relatively simple too. Starting with
p_ci_builds_metadata
will allow us to achieve tangible and quantifiable
results earlier, and will become a new pattern that makes partitioning the
largest table possible. We will partition builds metadata using the LIST
partitioning strategy.
Once we have many partitions attached to p_ci_builds_metadata
, with many
partition_ids
we will choose another CI table to partition next. In that case
we might want to use RANGE
partitioning in for that next table because
p_ci_builds_metadata
will already have many physical partitions, and
therefore many logical partition_ids
will be used at that time. For example,
if we choose ci_builds
as the next partitioning candidate, after having
partitioned p_ci_builds_metadata
, it will have many different values stored
in ci_builds.partition_id
. Using RANGE
partitioning in that case might be
easier.
Physical partitioning and logical partitioning will be separated, and a
strategy will be determined when we implement physical partitioning for the
respective database tables. Using RANGE
partitioning works similarly to using
LIST
partitioning in database tables, but because we can guarantee continuity
of partition_id
values, using RANGE
partitioning might be a better
strategy.
Multi-project pipelines
Parent-child pipeline will always be part of the same partition because child pipelines are considered a resource of the parent pipeline. They can’t be viewed individually in the project pipeline list page.
On the other hand, multi-project pipelines can be viewed in the pipeline list page.
They can also be accessed from the pipeline graph as downstream/upstream links
when created via the trigger
token or the API using a job token.
They can also be created from other pipelines by using trigger tokens, but in this
case we don’t store the source pipeline.
While partitioning ci_builds
we need to update the foreign keys to the
ci_sources_pipelines
table:
Foreign-key constraints:
"fk_be5624bf37" FOREIGN KEY (source_job_id) REFERENCES ci_builds(id) ON DELETE CASCADE
"fk_d4e29af7d7" FOREIGN KEY (source_pipeline_id) REFERENCES ci_pipelines(id) ON DELETE CASCADE
"fk_e1bad85861" FOREIGN KEY (pipeline_id) REFERENCES ci_pipelines(id) ON DELETE CASCADE
A ci_sources_pipelines
record references two ci_pipelines
rows (parent and
the child). Our usual strategy has been to add a partition_id
to the
table, but if we do it here we will force all multi-project
pipelines to be part of the same partition.
We should add two partition_id
columns for this table, a
partition_id
and a source_partition_id
:
Foreign-key constraints:
"fk_be5624bf37" FOREIGN KEY (source_job_id, source_partition_id) REFERENCES ci_builds(id, source_partition_id) ON DELETE CASCADE
"fk_d4e29af7d7" FOREIGN KEY (source_pipeline_id, source_partition_id) REFERENCES ci_pipelines(id, source_partition_id) ON DELETE CASCADE
"fk_e1bad85861" FOREIGN KEY (pipeline_id, partition_id) REFERENCES ci_pipelines(id, partition_id) ON DELETE CASCADE
This solution is the closest to a two way door decision because:
- We retain the ability to reference pipelines in different partitions.
- If we later decide that we want to force multi-project pipelines in the same partition we could add a constraint to validate that both columns have the same value.
Why do we want to use explicit logical partition ids?
Partitioning CI/CD data using a logical partition_id
has several benefits. We
could partition by a primary key, but this would introduce much more complexity
and additional cognitive load required to understand how the data is being
structured and stored in partitions.
CI/CD data is hierarchical data. Stages belong to pipelines, builds belong to
stages, artifacts belong to builds (with rare exceptions). We are designing a
partitioning strategy that reflects this hierarchy, to reduce the complexity
and therefore cognitive load for contributors. With an explicit partition_id
associated with a pipeline, we can cascade the partition ID number when trying
to retrieve all resources associated with a pipeline. We know that for a
pipeline 12345
with a partition_id
of 102
, we are always able to find
associated resources in logical partitions with number 102
in other routing
tables, and PostgreSQL will know in which partitions these records are being
stored in for every table.
Another interesting benefit for using a single and incremental latest
partition_id
number, associated with pipelines, is that in theory we can
cache it in Redis or in memory to avoid excessive reads from the database to
find this number, though we might not need to do this.
The single and uniform partition_id
value for pipeline data gives us more
choices later on than primary-keys-based partitioning.
Altering partitioned tables
It will still be possible to run ALTER TABLE
statements against partitioned tables,
similarly to how the tables behaved before partitioning. When PostgreSQL runs
an ALTER TABLE
statement against a parent partitioned table, it acquires the same
lock on all child partitions and updates each to keep them in sync. This differs from
running ALTER TABLE
on a non-partitioned table in a few key ways:
- PostgreSQL acquires
ACCESS EXCLUSIVE
locks against a larger number of tables, but not a larger amount of data, than it would were the table not partitioned. Each partition will be locked similarly to the parent table, and all will be updated in a single transaction. - Lock duration will be increased based on the number of partitions involved.
All
ALTER TABLE
statements executed on the GitLab database (other thanVALIDATE CONSTRAINT
) take small constant amounts of time per table modified. PostgreSQL will need to modify each partition in sequence, increasing the runtime of the lock. This time will still remain very small until there are many partitions involved. - If thousands of partitions are involved in an
ALTER TABLE
, we will need to verify that the value ofmax_locks_per_transaction
is high enough to support all of the locks that need to be taken during the operation.
Splitting large partitions into smaller ones
We want to start with the initial partition_id
number 100
(or higher, like
1000
, depending on our calculations and estimations). We do not want to start
from 1, because existing tables are also large already, and we might want to
split them into smaller partitions. If we start with 100
, we will be able to
create partitions for partition_id
of 1
, 20
, 45
, and move existing
records there by updating partition_id
from 100
to a smaller number.
PostgreSQL will move these records into their respective partitions in a consistent way, provided that we do it in a transaction for all pipeline resources at the same time. If we ever decide to split large partitions into smaller ones (it’s not yet clear if we will need to do this), we might be able to just use background migrations to update partition IDs, and PostgreSQL is smart enough to move rows between partitions on its own.
Naming conventions
A partitioned table is called a routing table and it will use the p_
prefix which should help us with building automated tooling for query analysis.
A table partition will be called partition and it can use the a physical
partition ID as suffix, for example ci_builds_101
. Existing CI tables will
become zero partitions of the new routing tables. Depending on the chosen
partitioning strategy for a given
table, it is possible to have many logical partitions per one physical partition.
Attaching first partition and acquiring locks
We learned when partitioning
the first table that PostgreSQL
requires an AccessExclusiveLock
on the table and
all of the other tables that it references through foreign keys. This can cause a deadlock
if the migration tries to acquire the locks in a different order from the application
business logic.
To solve this problem, we introduced a priority locking strategy to avoid further deadlock errors. This allows us to define the locking order and then try keep retrying aggressively until we acquire the locks or run out of retries. This process can take up to 40 minutes.
With this strategy, we successfully acquired a lock on ci_builds
table after 15 retries
during a low traffic period(after 00:00 UTC
).
See an example of this strategy in our partition tooling).
Partitioning steps
The database partition tooling
docs contain a list of steps to partition a table, but the steps are not enough
for our iterative strategy. As our dataset continues to grow we want to take
advantage of partitioning performance right away and not wait until all tables
are partitioned. For example, after partitioning the ci_builds_metadata
table
we want to start writing and reading data to/from a new partition. This means
that we will increase the partition_id
value from 100
, the default value,
to 101
. Now all of the new resources for the pipeline hierarchy will be
persisted with partition_id = 101
. We can continue following the database
tooling instructions for the next table that will be partitioned, but we require
a few extra steps:
- add
partition_id
column for the FK references with default value of100
since the majority of records should have that value. - change application logic to cascade the
partition_id
value -
correct
partition_id
values for recent records with a post deploy/background migration, similar to this:UPDATE ci_pipeline_metadata SET partition_id = ci_pipelines.partition_id FROM ci_pipelines WHERE ci_pipelines.id = ci_pipeline_metadata.pipeline_id AND ci_pipelines.partition_id in (101, 102);
- change the foreign key definitions
- …
Storing partitions metadata in the database
To build an efficient mechanism that will be responsible for creating
new partitions, and to implement time decay we want to introduce a partitioning
metadata table, called ci_partitions
. In that table we would store metadata
about all the logical partitions, with many pipelines per partition. We may
need to store a range of pipeline ids per logical partition. Using it we will
be able to find the partition_id
number for a given pipeline ID and we will
also find information about which logical partitions are “active” or
“archived”, which will help us to implement a time-decay pattern using database
declarative partitioning.
Doing that will also allow us to use a Unified Resource Identifier for
partitioned resources, that will contain a pointer to a pipeline ID, we could
then use to efficiently lookup a partition the resource is stored in. It might
be important when a resources can be directly referenced by an URL, in UI or
API. We could use an ID like 1e240-5ba0
for pipeline 123456
, build 23456
.
Using a dash -
can prevent an identifier from being highlighted and copied
with a mouse double-click. If we want to avoid this problem, we can use any
character of written representation that is not present in base-16 numeral
system - any letter from g
to z
in Latin alphabet, for example x
. In that
case an example of an URI would look like 1e240x5ba0
. If we decide to update
the primary identifier of a partitioned resource (today it is just a big
integer) it is important to design a system that is resilient to migrating data
between partitions, to avoid changing identifiers when rebalancing happens.
ci_partitions
table will store information about a partition identifier,
pipeline ids range it is valid for and whether the partitions have been
archived or not. Additional columns with timestamps may be helpful too.
Implementing a time-decay pattern using partitioning
We can use ci_partitions
to implement a time-decay pattern using declarative
partitioning. By telling PostgreSQL which logical partitions are archived we
can stop reading from these partitions using a SQL query like the one below.
SELECT * FROM ci_builds WHERE partition_id IN (
SELECT id FROM ci_partitions WHERE active = true
);
This query will make it possible to limit the number of partitions we will read from, and therefore will cut access to “archived” pipeline data, using our data retention policy for CI/CD data. Ideally we do not want to read from more than two partitions at once, so we need to align the automatic partitioning mechanisms with the time-decay policy. We will still need to implement new access patterns for the archived data, presumably through the API, but the cost of storing archived data in PostgreSQL will be reduced significantly this way.
There are some technical details here that are out of the scope of this description, but by using this strategy we can “archive” data, and make it much less expensive to reside in our PostgreSQL cluster by toggling a boolean column value.
Accessing partitioned data
It will be possible to access partitioned data whether it has been archived or
not, in most places in GitLab. On a merge request page, we will always show
pipeline details even if the merge request was created years ago. We can do
that because ci_partitions
will be a lookup table associating a pipeline ID
with its partition_id
, and we will be able to find the partition that the
pipeline data is stored in.
We will need to constrain access to searching through pipelines, builds, artifacts etc. Search cannot be done through all partitions, as it would not be efficient enough, hence we will need to find a better way of searching through archived pipelines data. It will be necessary to have different access patterns to access archived data in the UI and API.
There are a few challenges in enforcing usage of the partition_id
partitioning key in PostgreSQL. To make it easier to update our application to
support this, we have designed a new queries analyzer in our
proof of concept merge request.
It helps to find queries that are not using the partitioning key.
In a separate proof of concept merge request
and related issue we
demonstrated that using the uniform partition_id
makes it possible to extend
Rails associations with an additional scope modifier so we can provide the
partitioning key in the SQL query.
Using instance dependent associations, we can easily append a partitioning key to SQL queries that are supposed to retrieve associated pipeline resources, for example:
has_many :builds, -> (pipeline) { where(partition_id: pipeline.partition_id) }
The problem with this approach is that it makes preloading much more difficult as instance dependent associations cannot be used with preloads:
ArgumentError: The association scope 'builds' is instance dependent (the
scope block takes an argument). Preloading instance dependent scopes is not
supported.
Query analyzers
We implemented 2 query analyzers to detect queries that need to be fixed so that everything keeps working with partitioned tables:
- One analyzer to detect queries not going through a routing table.
- One analyzer to detect queries that use routing tables without specifying the
partition_id
in theWHERE
clauses.
We started by enabling our first analyzer in test
environment to detect existing broken
queries. It is also enabled on production
environment, but for a small subset of the traffic (0.1%
)
because of scalability concerns.
The second analyzer will be enabled in a future iteration.
Primary key
Primary key must include the partitioning key column to partition the table.
We first create a unique index including the (id, partition_id)
.
Then, we drop the primary key constraint and use the new index created to set
the new primary key constraint.
ActiveRecord
does not support
composite primary keys, so we must force it to treat the id
column as a primary key:
class Model < ApplicationRecord
self.primary_key = 'id'
end
The application layer is now ignorant of the database structure and all of the
existing queries from ActiveRecord
continue to use the id
column to access
the data. There is some risk to this approach because it is possible to
construct application code that results in duplicate models with the same id
value, but on a different partition_id
. To mitigate this risk we must ensure
that all inserts use the database sequence to populate the id
since they are
guaranteed
to allocate distinct values and rewrite the access patterns to include the
partition_id
value. Manually assigning the ids during inserts must be avoided.
Foreign keys
Foreign keys must reference columns that either are a primary key or form a unique constraint. We can define them using these strategies:
Between routing tables sharing partition ID
For relations that are part of the same pipeline hierarchy it is possible to
share the partition_id
column to define the foreign key constraint:
p_ci_pipelines:
- id
- partition_id
p_ci_builds:
- id
- partition_id
- pipeline_id
In this case, p_ci_builds.partition_id
indicates the partition for the build
and also for the pipeline. We can add a FK on the routing table using:
ALTER TABLE ONLY p_ci_builds
ADD CONSTRAINT fk_on_pipeline_and_partition
FOREIGN KEY (pipeline_id, partition_id)
REFERENCES p_ci_pipelines(id, partition_id) ON DELETE CASCADE;
Between routing tables with different partition IDs
It’s not possible to reuse the partition_id
for all relations in the CI domain,
so in this case we’ll need to store the value as a different attribute. For
example, when canceling redundant pipelines we store on the old pipeline row
the ID of the new pipeline that cancelled it as auto_canceled_by_id
:
p_ci_pipelines:
- id
- partition_id
- auto_canceled_by_id
- auto_canceled_by_partition_id
In this case we can’t ensure that the canceling pipeline is part of the same
hierarchy as the canceled pipelines, so we need an extra attribute to store its
partition, auto_canceled_by_partition_id
, and the FK becomes:
ALTER TABLE ONLY p_ci_pipelines
ADD CONSTRAINT fk_cancel_redundant_pipelines
FOREIGN KEY (auto_canceled_by_id, auto_canceled_by_partition_id)
REFERENCES p_ci_pipelines(id, partition_id) ON DELETE SET NULL;
Between routing tables and regular tables
Not all of the tables in the CI domain will be partitioned, so we’ll have routing
tables that will reference non-partitioned tables, for example we reference
external_pull_requests
from ci_pipelines
:
FOREIGN KEY (external_pull_request_id)
REFERENCES external_pull_requests(id)
ON DELETE SET NULL
In this case we only need to move the FK definition from the partition level to the routing table so that new pipeline partitions may use it:
ALTER TABLE p_ci_pipelines
ADD CONSTRAINT fk_external_request
FOREIGN KEY (external_pull_request_id)
REFERENCES external_pull_requests(id) ON DELETE SET NULL;
Between regular tables and routing tables
Most of the tables from the CI domain reference at least one table that will be
turned into a routing tables, for example ci_pipeline_messages
references
ci_pipelines
. These definitions will need to be updated to use the routing
tables and for this they will need a partition_id
column:
p_ci_pipelines:
- id
- partition_id
ci_pipeline_messages:
- id
- pipeline_id
- pipeline_partition_id
The foreign key can be defined by using:
ALTER TABLE ci_pipeline_messages ADD CONSTRAINT fk_pipeline_partitioned
FOREIGN KEY (pipeline_id, pipeline_partition_id)
REFERENCES p_ci_pipelines(id, partition_id) ON DELETE CASCADE;
The old FK definition will need to be removed, otherwise new inserts in the
ci_pipeline_messages
with pipeline IDs from non-zero partition will fail with
reference errors.
Indexes
We learned that PostgreSQL
does not allow to create a single index (unique or otherwise) across all partitions of a table.
One solution to solve this problem is to embed the partitioning key inside the uniqueness constraint.
This might mean prepending the partition ID in a hexadecimal format before the token itself and storing the concatenated string in a database. To do that we would need to reserve an appropriate number of leading bytes in a token to accommodate for the maximum number of partitions we may have in the future. It seems that reserving four characters, what would translate into 16-bits number in base-16, might be sufficient. The maximum number we can encode this way would be FFFF, what is 65535 in decimal.
This would provide a unique constraint per-partition which is sufficient for global uniqueness.
We have also designed a query analyzer that makes it possible to detect direct
usage of zero partitions, legacy tables that have been attached as first
partitions to routing tables, to ensure that all queries are targeting
partitioned schema or partitioned routing tables, like p_ci_pipelines
.
Why not partition using the project or namespace ID?
We do not want to partition using project_id
or namespace_id
because
sharding and podding is a different problem to solve, on a different layer of
the application. It doesn’t solve the original problem statement of performance
growing worse over time as we build up infrequently read data. We may want to
introduce pods in the future, and that might become the primary mechanism of
separating data based on the group or project the data is associated with.
In theory we could use either project_id
or namespace_id
as a second
partitioning dimension, but this would add more complexity to a problem that is
already very complex.
Partitioning builds queuing tables
We also want to partition our builds queuing tables. We currently have two:
ci_pending_builds
and ci_running_builds
. These tables are different from
other CI/CD data tables, as there are business rules in our product that make
all data stored in them invalid after 24 hours.
As a result, we will need to use a different strategy to partition those database tables, by removing partitions entirely after these are older than 24 hours, and always reading from two partitions through a routing table. The strategy to partition these tables is well understood, but requires a solid Ruby-based automation to manage the creation and deletion of these partitions. To achieve that we will collaborate with the Database team to adapt existing database partitioning tools to support CI/CD data partitioning.
Iterating to reduce the risk
This strategy should reduce the risk of implementing CI/CD partitioning to acceptable levels. We are also focusing on implementing partitioning for reading only from two partitions initially to make it possible to detach zero partitions in case of problems in our production environment. Every iteration phase, described below has a revert strategy and before shipping database changes we want to test them in our benchmarking environment.
The main way of reducing risk in case of this effort is iteration and making things reversible. Shipping changes, described in this document, in a safe and reliable way is our priority.
As we move forward with the implementation we will need to find even more ways to iterate on the design, support incremental rollouts and have better control over reverting changes in case of something going wrong. It is sometimes challenging to ship database schema changes iteratively, and even more difficult to support incremental rollouts to the production environment. This can, however, be done, it just sometimes requires additional creativity, that we will certainly need here. Some examples of how this could look like:
Incremental rollout of partitioned schema
Once we introduce a first partitioned routing table (presumably
p_ci_pipelines
) and attach its zero partition (ci_pipelines
), we will need
to start interacting with the new routing table, instead of a concrete
partition zero. Usually we would override the database table the Ci::Pipeline
Rails model would use with something like self.table_name = 'p_ci_pipelines'
.
Unfortunately this approach might not support incremental rollout, because
self.table_name
will be read upon application boot up, and later we might be
unable revert this change without restarting the application.
One way of solving this might be introducing Ci::Partitioned::Pipeline
model,
that will inherit from Ci::Pipeline
. In that model we would set
self.table_name
to p_ci_pipeline
and return its meta class from
Ci::Pipeline.partitioned
as a scope. This will allow us to use feature flags
to route reads from ci_pipelines
to p_ci_pipelines
with a simple revert
strategy.
Incremental experimentation with partitioned reads
Another example would be related to the time when we decide to attach another
partition. The goal of Phase 1 will be have two partitions per partitioned
schema / routing table, meaning that for p_ci_pipelines
we will have
ci_pipelines
attached as partition zero, and a new ci_pipelines_p1
partition created for new data. All reads from p_ci_pipelines
will also need
to read data from the p1
partition and we should also iteratively experiment
with reads targeting more than one partition, to evaluate performance and
overhead of partitioning.
We can do that by moving old data to ci_pipelines_m1
(minus 1) partition
iteratively. Perhaps we will create partition_id = 1
and move some really old
pipelines there. We can then iteratively migrate data into m1
partition to
measure the impact, performance and increase our confidence before creating a
new partition p1
for new (still not created) data.
Iterations
We want to focus on Phase 1 iteration first. The goal and the main objective of this iteration is to partition the biggest 6 CI/CD database tables into 6 routing tables (partitioned schema) and 12 partitions. This will leave our Rails SQL queries mostly unchanged, but it will also make it possible to perform emergency detachment of “zero partitions” if there is a database performance degradation. This will cut users off their old data, but the application will remain up and running, which is a better alternative to application-wide outage.
- Phase 0: Build CI/CD data partitioning strategy: Done. ✅
-
Phase 1: Partition the 6 biggest CI/CD database tables.
- Create partitioned schemas for all 6 database tables.
- Design a way to cascade
partition_id
to all partitioned resources. - Implement initial query analyzers validating that we target routing tables.
- Attach zero partitions to the partitioned database tables.
- Update the application to target routing tables and partitioned tables.
- Measure the performance and efficiency of this solution.
Revert strategy: Switch back to using concrete partitions instead of routing tables.
-
Phase 2: Add a partitioning key to add SQL queries targeting partitioned tables.
- Implement query analyzer to check if queries targeting partitioned tables are using proper partitioning keys.
- Modify existing queries to make sure that all of them are using a partitioning key as a filter.
Revert strategy: Use feature flags, query by query.
-
Phase 3: Build new partitioned data access patterns.
- Build a new API or extend an existing one to allow access to data stored in partitions that are supposed to be excluded based on the time-decay data retention policy.
Revert strategy: Feature flags.
-
Phase 4: Introduce time-decay mechanisms built on top of partitioning.
- Build time-decay policy mechanisms.
- Enable the time-decay strategy on GitLab.com.
-
Phase 5: Introduce mechanisms for creating partitions automatically.
- Make it possible to create partitions in an automatic way.
- Deliver the new architecture to self-managed instances.
The diagram below visualizes this plan on Gantt chart. The dates on the chart below are just estimates to visualize the plan better, these are not deadlines and can change at any time.
Conclusions
We want to build a solid strategy for partitioning CI/CD data. We are aware of the fact that it is difficult to iterate on this design, because a mistake made in managing the database schema of our multi-terabyte PostgreSQL instance might not be easily reversible without potential downtime. That is the reason we are spending a significant amount of time to research and refine our partitioning strategy. The strategy, described in this document, is subject to iteration as well. Whenever we find a better way to reduce the risk and improve our plan, we should update this document as well.
We’ve managed to find a way to avoid large-scale data migrations, and we are building an iterative strategy for partitioning CI/CD data. We documented our strategy here to share knowledge and solicit feedback from other team members.
Who
DRIs:
Role | Who |
---|---|
Author | Grzegorz Bizon, Principal Engineer |
Recommender | Kamil Trzciński, Senior Distinguished Engineer |
Product Leadership | Jackie Porter, Director of Product Management |
Engineering Leadership | Caroline Simpson, Engineering Manager / Cheryl Li, Senior Engineering Manager |
Lead Engineer | Marius Bobin, Senior Backend Engineer |
Senior Engineer | Maxime Orefice, Senior Backend Engineer |
Senior Engineer | Tianwen Chen, Senior Backend Engineer |