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Time-Decay Data

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Time-Decay data

This document describes the time-decay pattern introduced in the Database Scalability Working Group. We discuss the characteristics of time-decay data and propose best practices for GitLab development to consider in this context.

Some datasets are subject to strong time-decay effects, in which recent data is accessed far more frequently than older data. Another aspect of time-decay is that with time some types of data become less important, which means that we can also move old data to a bit less durable/available storage or even delete them in extreme cases.

Those effects are usually tied to product and/or application semantics and can vary in the degree that older data are accessed and how useful or required older data are to the users or the application.

Let's first consider entities with no inherent time related bias for their data. A record for a user or a project may be equally important and frequently accessed irrelevant to when it was created. We can not predict by using a user's id or created_at how often the related record will be accessed or updated.

On the other side, a good example for datasets with extreme time-decay effects are logs and time series data, like, for example, events recording user actions. Most of the times that type of data may have no business use after a couple of days or weeks and quickly become less important even from a data analysis perspective. They represent a snapshot that is very quickly becoming less and less relevant to the current state of the application, until at some point they have no real value.

In the middle of the two extremes we can find datasets that have useful information that we want to keep around, but with old records seldomly being accessed after an initial (small) time period after they are created.

Characteristics of time-decay data

We are interested in datasets that show the following characteristics:

  1. Size of the dataset: They are considerably large.
  2. Access methods: We can filter the vast majority of queries accessing the dataset by a time related dimension or a categorial dimension with time decay effects.
  3. Immutability: The time-decay status does not change.
  4. Retention: whether we want to keep the old data or not and/or whether old data will be accessible by users through the application.

Size of the dataset

There can be datasets of variable sizes that show strong time-decay effects, but in the context of this blueprint we are going to focus on entities with a considerably large dataset.

Smaller datasets do not contribute significantly to the database related resource usage, nor do they inflict a considerable performance penalty to queries.

In contrast, large datasets over ~50 Million records and/or 100GB in size add a significant overhead to constantly accessing a really small subset of the data. In those cases we would want to use the time-decay effect in our advantage and reduce the actively accessed dataset.

Access methods

The second and most important characteristic of time-decay data is that most of the times we are able to implicitly or explicitly access the data using a date filter, restricting our results based on a time related dimension.

There can be many such dimensions, but we are only going to focus on the creation date as it is both the most commonly used and the one that we can control and optimize against. It is immutable, set when the record is created, and can be tied to physically clustering the records without having to move them around.

It's important to add that even if time-decay data are not accessed that way by the application by default, there is a way to make the vast majority of the queries explicitly filter the data in such a way. Time decay data without such a time-decay related access method are of no use from an optimization perspective as there is no way to set and follow a scaling pattern.

We are not restricting the definition to data that are always accessed using a time-decay related access method, as there may be some outlier operations, which may be necessary and we can accept them not scaling that well as long as the rest of the accesss methods can scale. An example could be an administrator accessing all past events of a specific type while all other operations only access a maximum of a month of events, restricted to 6 months in the past.

Immutability

The third characteristic of time-decay data is that their time-decay status does not change. Once they are considered "old", they can not switch back to "new" or relevant again.

This definition may sound trivial but we have to be able to make operations over "old" data more expensive (e.g. by archiving or moving them to less expensive storage) without having to worry about the repercussions of switching back to being relevant and having important application operations underperforming.

Consider as a counter example to a time-decay data access pattern an application view that presents issues by when they were updated. We are also interested in the most recent data from an "update" perspective, but that definition is volatile and not actionable.

Retention

Finally, a characteristic that further differentiates time-decay data in sub categories with slightly different approaches available is whether we want to keep the old data or not (e.g. retention policy) and/or whether old data will be accessible by users through the application.

(optional) Extended definition of time-decay data

As a side note, if we extend the aforementioned definitions to access patterns that restrict access to a well defined subset of the data based on a clustering attribute, we could use the time-decay scaling patterns for many other types of data.

As an example, consider data that are only accessed while they are labeled as active, like ToDos not marked as done, pipelines for not merged MRs (or a similar not time based constraint), etc. In this case, instead of using a time dimension to define the decay, we use a categorical dimension (i.e. one that uses a finite set of values) to define the subset of interest. As long as that subset is small compared to the overall size of the dataset, we could use the same approach.

Similarly, we may define data as old based both on a time dimension and additional status attributes, like for example ci pipelines that have failed more than 6 months ago.

Time-Decay data strategies

Partitioning

This is the acceptable best practice for addressing time-decay data from a pure database perspective. You can find more information on table partitioning for PostgreSQL in the documentation page for table partitioning.

Partitioning by date intervals (e.g. month, year) allows us to create much smaller tables (partitions) for each date interval and only access the most recent partition(s) for any application related operation.

We have to set the partitioning key based on the date interval of interest, which may depend on two factors:

  1. How far back in time do we need to access data for?

    Partitioning by week is of no use if we always access data for a year back, as we would have to execute queries over 52 different partitions (tables) each time. As an example for that consider the activity feed on the profile of any GitLab user.

    In contrast, if we want to just access the last 7 days of created records, partitioning by year would include too many unnecessary records in each partition, as is the case for web_hook_logs.

  2. How large are the partitions created?

    The major purpose of partitioning is accessing tables that are as small as possible. If they get too large by themselves, queries will start underperforming and we may have to re-partition (split) them in even smaller partitions.

The perfect partitioning scheme keeps all queries over a dataset almost always over a single partition, with some cases going over two partitions and seldomly over multiple partitions being an acceptable balance. We should also target for partitions that are as small as possible, bellow 5-10M records and/or 10GB each maximum.

Partitioning can be combined with other strategies to either prune (drop) old partitions, move them to cheaper storage inside the database or move them outside of the database (archive or use of other types of storage engines).

As long as we do not want to keep old records and partitioning is used, pruning old data has a constant, for all intents and purposes zero, cost compared to deleting the data from a huge table (as described in the following sub-section). We just need a background worker to drop old partitions whenever all the data inside that partition get out of the retention policy's period.

As an example, if we only want to keep records no more than 6 months old and we partition by month, we can safely keep the 7 latest partitions at all times (current month and 6 months in the past). That means that we can have a worker dropping the 8th oldest partition at the start of each month.

Moving partitions to cheaper storage inside the same database is relatively simple in PostgreSQL through the use of tablespaces. It is possible to specify a tablespace and storage parameters for each partition separately, so the approach in this case would be to:

Finally, moving partitions outside of the database can be achieved through database archiving or manually exporting the partitions to a different storage engine (more details in the dedicated sub-section).

Pruning old data

If we don't want to keep old data around in any form, we can implement a pruning strategy and delete old data.

It's a simple to implement strategy that uses a pruning worker to delete past data. As an example that we'll further analyze below, we are pruning old web_hook_logs older than 90 days.

The disadvantage of such a solution over large, non-partitioned tables is that we have to manually access and delete all the records that are considered as not relevant any more. That is a very expensive operation due to multi-version concurrency control in Postgres. It also leads to the pruning worker not being able to catchup with new records being created if that rate exceeds a threshold, as is the case of web_hook_logs at the time of writing this document.

For the aforementioned reasons, our proposal is that we should base any implementation of a data retention strategy on partitioning, unless there are strong reasons not to.

Moving old data outside of the database

In most cases, we consider old data as valuable, so we do not want to prune them. If at the same time, they are not required for any database related operations (e.g. directly accessed or used in joins and other types of queries), we can move them outside of the database.

That does not mean that they are not directly accessible by users through the application; we could move data outside the database and use other storage engines or access types for them, similarly to offloading metadata but only for the case of old data.

In the simplest use case we can provide fast and direct access to recent data, while allowing users to download an archive with older data. This is an option evaluated in the audit_events use case; depending on the country and industry, audit events may have a very long retention period, while only the past month(s) of data are actively accessed through GitLab's interface.

Additional use cases may include exporting data to a datawarehouse or other types of data stores as they may be better suited for processing that type of data. An example can be JSON logs that we sometimes store in tables: loading such data into a BigQuery or a columnar store like Redshift may be better for analyzing/querying the data.

We might consider a number of strategies for moving data outside of the database:

  1. Streaming this type of data into logs and then move them to secondary storage options or load them to other types of data stores directly (as CSV/JSON data).
  2. Creating an ETL process that exports the data to CSV, uploads them to object storage, drops this data from the database, and then loads the CSV into a different data store.
  3. Loading the data in the background by using the API provided by the data store.

    This may be a not viable solution for large datasets; as long as bulk uploading using files is an option, it should outperform API calls.

Use cases

Web hook logs

Related epic: Partitioning: web_hook_logs table

The important characteristics of web_hook_logs are the following:

  1. Size of the dataset: It is a really large table. At the moment we decided to partition it (2021-03-01), it had ~527M records and a total size of ~1TB

    Table Rows Total size Table size Index(es) Size TOAST Size
    web_hook_logs ~527M 1.02 TiB (10.46%) 713.02 GiB (13.37%) 42.26 GiB (1.10%) 279.01 GiB (38.56%)
  2. Access methods: We always request for the past 7 days of logs at max.
  3. Immutability: It can be partitioned by created_at, an attribute that does not change.
  4. Retention: There is a 90 days retention policy set for it.

Additionally, we were at the time trying to prune the data by using a background worker (PruneWebHookLogsWorker), which could not keep up with the rate of inserts.

As a result, on March 2021 there were still not deleted records since July 2020 and the table was increasing in size by more than 2 million records per day instead of staying at a more or less stable size.

Finally, the rate of inserts has grown to more than 170GB of data per month by March 2021 and keeps on growing, so the only viable solution to pruning old data was through partitioning.

Our approach was to partition the table per month as it aligned with the 90 days retention policy.

The process required follows:

  1. Decide on a partitioning key

    Using the created_at column is straightforward in this case: it is a natural partitioning key when a retention policy exists and there were no conflicting access patterns.

  2. Once we decide on the partitioning key, we can start with creating the partitions and backfill them (copy data from the existing table).

    The reason for that is that we can't just partition an existing table, we have to create a new partitioned table.

    So, we have to create the partitioned table and all the related partitions, start copying everything over and also add sync triggers so that any new data or updates/deletes to existing data can be mirrored to the new partitioned table.

    MR with all the necessary details on how to start partitioning a table

    It required a 15 days and 7.6 hours to complete that process.

  3. Next step, one milestone after the initial partitioning starts is to clean up after the background migration used to backfill and finish executing any remaining jobs, retry failed jobs, etc.

    MR with all the necessary details

  4. Then we can add any remaining foreign keys and secondary indexes to the partitioned table.

    The purpose of this operation is to bring its schema on par with the original non partitioned table before we can swap them in the next milestone.

    We are not adding them at the beginning as they are adding overhead to each insert and they would slow down the initial backfilling of the table (in this case for more than half a billion records, which can add up significantly). So we create a lightweight, vanilla version of the table, copy all the data and then add any remaining indexes and foreign keys.

    MRs with all the necessary details: MRs adding indexes(MR-1, MR-2), MR adding Foreign Keys

  5. Swap the base table with partitioned copy

    This is the point when the partitioned table starts actively being used by the application.

    Dropping the original table is a destructive operation and we want to make sure that we had no issues during the process, so we keep the old non-partitioned table. We also switch the sync trigger the other way around so that the non-partitioned table is still up to date with any operations happening on the partitioned table. That allows us to swap back the tables if it is necessary.

    MR with all the necessary details

  6. Last step, one milestone after the swap - Drop the non-partitioned table

    Issue with all the necessary details

  7. After the non-partitioned table is dropped, we can add a worker to implement the pruning strategy by dropping past partitions.

    In this case, the worker will make sure that only 4 partitions are always active (as the retention policy is 90 days) and drop any partitions older than four months. We have to keep 4 months of partitions while the current month is still active, as going 90 days back takes you to the fourth oldest partition.

Audit Events

Related epic: Partitioning: Design and implement partitioning strategy for Audit Events

The audit_events table shares a lot of characteristics with the web_hook_logs table discussed in the previous sub-section, so we are going to focus on the points they differ.

There was a need for a solution (1, 2) and a consensus that partitioning could solve most of the performance issues (1, 2).

In contrast to most other large tables, it has no major conflicting access patterns: we could switch the access patterns to align with partitioning by month. This is not the case for example for other tables, which even though could justify a partitioning approach (e.g. by namespace), they have many conflicting access patterns.

In addition, audit_events is a write heavy table with very few reads (queries) over it and has a very simple schema, not connected with the rest of the database (no incoming or outgoing FK constraints) and with only two indexes defined over it.

The later was important at the time as not having Foreign Key constraints meant that we could partition it while we were still in PostgreSQL 11. This is not a concern any more now that we have moved to PostgreSQL 12 as a required default, as can be seen for the web_hook_logs use case above.

The migrations and steps required for partitioning the audit_events are similar to the ones described in the previous sub-section for web_hook_logs. There is no retention strategy defined for audit_events at the moment, so there is no pruning strategy implemented over it, but we may implement an archiving solution in the future.

What's interesting on the case of audit_events is the discussion on the necessary steps that we had to follow to implement the UI/UX Changes needed to encourage optimal querying of the partitioned. It can be used as a starting point on the changes required on the application level to align all access patterns with a specific time-decay related access method.

CI tables

WIP: Requirements and analysis of the CI tables use case - Still a work in progress, so we'll have to add more details after the analysis moves forward.

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