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TEP-0127: Larger Results via Sidecar Logs
Today, `Results` have a size limit of 4KB per `Step` and 12KB per `TaskRun` in the best case - see [issue][4012]. The goal of [TEP-0086][tep-0086] is to support larger `Results` beyond the current size limits. TEP-0086 has many alternatives but no proposal. This TEP is proposes experimenting with one of the alternatives - `Sidecar` logs. This allows us to support larger `Results` that are stored within `TaskRun` CRDs. [4012]: tektoncd/pipeline#4012 [4808]: tektoncd/pipeline#4808 [tep-0086]: https://github.com/tektoncd/community/blob/main/teps/0086-changing-the-way-result-parameters-are-stored.md
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| --- | ||
| status: implementable | ||
| title: Larger Results via Sidecar Logs | ||
| creation-date: '2022-11-30' | ||
| last-updated: '2022-11-30' | ||
| authors: | ||
| - '@chitrangpatel' | ||
| - '@jerop' | ||
| see-also: | ||
| - TEP-0086 | ||
| --- | ||
|
|
||
| # TEP-0127: Larger Results via Sidecar Logs | ||
|
|
||
| <!-- toc --> | ||
| - [Summary](#summary) | ||
| - [Motivation](#motivation) | ||
| - [Goals](#goals) | ||
| - [Non-Goals](#non-goals) | ||
| - [Use Cases](#use-cases) | ||
| - [Proposal](#proposal) | ||
| - [Feature Gate](#feature-gate) | ||
| - [Logs Access](#logs-access) | ||
| - [Size Limit](#size-limit) | ||
| - [PipelineRun Status](#pipelinerun-status) | ||
| - [Design Details](#design-details) | ||
| - [Design Evaluation](#design-evaluation) | ||
| - [Reusability](#reusability) | ||
| - [Simplicity](#simplicity) | ||
| - [Performance](#performance) | ||
| - [Security](#security) | ||
| - [Experiment Questions](#experiment-questions) | ||
| - [References](#references) | ||
| <!-- /toc --> | ||
|
|
||
| This TEP builds on the hard work of many people who have been tackling the problem over the past couple years, | ||
| including but not limited to: | ||
| - '@abayer' | ||
| - '@afrittoli' | ||
| - '@bobcatfish' | ||
| - '@dibyom' | ||
| - '@imjasonh' | ||
| - '@pritidesai' | ||
| - '@ScrapCodes' | ||
| - '@skaegi' | ||
| - '@tlawrie' | ||
| - '@tomcli' | ||
| - '@vdemeester' | ||
| - '@wlynch' | ||
|
|
||
| ## Summary | ||
|
|
||
| Today, `Results` have a size limit of 4KB per `Step` and 12KB per `TaskRun` in the best case - see [issue][4012]. | ||
|
|
||
| The goal of [TEP-0086][tep-0086] is to support larger `Results` beyond the current size limits. TEP-0086 has many | ||
| alternatives but no proposal. This TEP is proposes experimenting with one of the alternatives - `Sidecar` logs. | ||
| This allows us to support larger `Results` which are stored within `TaskRun` CRDs. | ||
|
|
||
| ## Motivation | ||
|
|
||
| `Results` are too small - see [issue][4012]. The current implementation of `Results` involves parsing from disk and | ||
| storing as part of the `Termination Message` which has a limit of 4KB per `Container` and 12KB per `Pod`. As such, | ||
| the size limit of `Results` is 12KB per `TaskRun` and 4KB per `Step` at best. | ||
|
|
||
| To make matters worse, the limit is divided equally among all `Containers` in a `Pod` - see [issue][4808]. Therefore, | ||
| the more the `Steps` in a `Task` the less the size limit for `Results`. For example, if there are 12 `Steps` then each | ||
| has 1KB in `Termination Message` storage to produce `Results`. | ||
|
|
||
| [TEP-0086][tep-0086] aims to support larger `Results`. [TEP-0086][tep-0086] has many [alternatives][tep-0086-alts] with | ||
| no proposal because there's no obvious "best" solution that would meet all the requirements. | ||
|
|
||
| This TEP proposes experimenting with `Sidecar` logs to support larger `Results` that are stored within `TaskRun` CRDs. | ||
| This allows us to provide an immediate solution to the urgent needs of users, while not blocking pursuit of the other | ||
| alternatives. | ||
|
|
||
| ### Goals | ||
|
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||
| The main goal is to support larger `Results` that are limited by the size of a `TaskRun` CRD (1.5MB) via `Sidecar` logs. | ||
|
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||
| ### Non-Goals | ||
|
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||
| The following are out of scope for this TEP: | ||
|
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| 1. Solving use cases that requires really large `Results` beyond the size limit of a CRD (1.5MB). | ||
|
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| 2. Addressing other [alternatives][tep-0086-alts] for larger `Results` that are listed in [TEP-0086][tep-0086]. | ||
|
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||
| ### Use Cases | ||
|
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| 1. Store larger `Results`, such as JSON payloads from an HTTP call or a Cloud Event, that can be inspected later or | ||
| passed to other `Tasks` without the need to understand storage mechanisms. | ||
|
|
||
| 3. The [documented guidance][docs] is that `Results` are used for outputs less than 1KB while `Workspaces` are used | ||
| for larger data. | ||
|
|
||
| > As a general rule-of-thumb, if a `Result` needs to be larger than a kilobyte, you should likely use a `Workspace` | ||
| to store and pass it between `Tasks` within a `Pipeline`. | ||
|
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| Supporting larger `Results` upto the CRD limit allows users to reuse `Tasks` in more scenarios without having to | ||
| change the specification to use `Workspaces` upon hitting the current low size limit of 4KB per `TaskRun`. | ||
|
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||
| ## Proposal | ||
|
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| We propose using stdout logs from a dedicated `Sidecar` to return a json `Result` object to support larger `Results`. | ||
| The `Pipeline` controller would wait for the `Sidecar` to exit and then read the logs based on a particular query and | ||
| append `Results` to the `TaskRun`. | ||
|
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| We propose injecting the dedicated `Sidecar` alongside other `Steps`. The `Sidecar` will watch the `Results` paths of | ||
| the `Steps`. This `Sidecar` will output the name of the `Result` and its contents to stdout in a parsable pattern. The | ||
| `TaskRun` controller will access the stdout logs of the `Sidecar` then extract the `Results` and its contents. | ||
|
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| After the `Steps` have terminated, the `TaskRun` controller will write the `Results` from the logs of the `Sidecar` | ||
| instead of using the `Termination Message`, hence bypassing the 4KB limit. This method keeps the rest of the current | ||
| functionality identical and does not require any external storage mechanism. | ||
|
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||
| For further details, see the [demonstration][demo] of the [implementation][poc]. | ||
|
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||
| This provides an opportunity to experiment with this solution to provide `Results` within the CRDs as we continue to | ||
| explore other alternatives, including those that involve external storage. | ||
|
|
||
| ### Feature Gate | ||
|
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||
| This feature will be gated using a `result-extraction-method` feature flag. | ||
|
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| Users have to set `result-extraction-method` to `"sidecar-logs"` to enable the feature: | ||
| ```shell | ||
| kubectl patch cm feature-flags -n tekton-pipelines -p '{"data":{"result-extraction-method":"sidecar-logs"}}' | ||
| ``` | ||
|
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||
| This feature is disabled by default. When disabled, `Results` continue to pass through `Termination Message`. | ||
|
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||
| ### Logs Access | ||
|
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| This feature requires that the `Pipeline` controller has access to `Pod` logs. | ||
|
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| Users have to grant `get` access to all `pods/log` to the `Pipeline` controller: | ||
| ```shell | ||
| kubectl apply -f config/enable-log-access-to-controller/ | ||
| ``` | ||
|
|
||
| ### Size Limit | ||
|
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| The size limit per `Result` can be configured using the `max-result-size` feature flag, which takes the integer value | ||
| of the bytes. | ||
|
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||
| The `max-result-size` feature flag defaults to 4096 bytes. This ensures that we support existing `Tasks` with only one | ||
| `Result` that uses up the whole size limit of the `Termination Message`. | ||
|
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| If users want to set the size limit per `Result` to be something other than 4096 bytes, they can set `max-result-size` | ||
| by setting `max-result-size: <VALUE-IN-BYTES>`. The value set here cannot exceed the CRD size limit of 1.5MB. | ||
|
|
||
| ```shell | ||
| kubectl patch cm feature-flags -n tekton-pipelines -p '{"data":{"max-result-size":"<VALUE-IN-BYTES>"}}' | ||
| ``` | ||
|
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||
| Even though the size limit per `Result` is configurable, the size of `Results` are limited by CRD size limit of 1.5MB. | ||
| If the size of `Results` exceeds this limit, then the `TaskRun` will fail with a message indicating the size limit has | ||
| been exceeded. | ||
|
|
||
| ### PipelineRun Status | ||
|
|
||
| In [TEP-0100][tep-0100], we proposed changes to `PipelineRun` status to reduce the amount of information stored about | ||
| the status of `TaskRuns` and `Runs`. As a result, the `PipelineRun` status is set up to handle larger `Results` without | ||
| exacerbating the storage issues that were there before. | ||
|
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||
| For `ChildReferences` to be populated, the `embedded-status` must be set to `"minimal"`. We recommend that the minimal | ||
| embedded status - `ChildReferences` - is enabled while migration is ongoing until it becomes the only supported status. | ||
| This will ensure that larger `Results` from all its `TaskRuns` will not bloat the `PipelineRun` CRD. | ||
|
|
||
| ## Design Details | ||
|
|
||
| The `Sidecar` will run a binary that: | ||
| - receives argument for `Results`' paths and names which are identified from `taskSpec.results` field. This allows the | ||
| `Sidecar` to know the `Results` it needs to read. | ||
| - has `/tekton/run` volume mounted where status of each `Step` is written. | ||
| - periodically checks for `Step` status in the path `/tekton/run`. | ||
| - when all `Steps` have completed, it immediately parses all the `Results` in paths and prints them to stdout in a | ||
| parsable pattern. | ||
|
|
||
| For further details, see the [demonstration][demo] of the [implementation][poc]. | ||
|
|
||
| ## Design Evaluation | ||
|
|
||
| ### Reusability | ||
|
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| This proposal does not introduce any API changes to specification `Results`. The changes are in implementation details | ||
| of `Results`. The existing `Tasks` will continue to function as they are, only that they can support larger `Results`. | ||
|
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||
| Even more, supporting larger `Results` upto the CRD limit allows users to reuse `Tasks` in more scenarios without | ||
| having to change the specification to use `Workspaces` upon hitting the current low size limit of 4KB per `TaskRun`. | ||
| This allows users to control execution as needed by their context without having to modify `Tasks` and `Pipelines`. | ||
|
|
||
| ### Simplicity | ||
|
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| This proposal provides a simple solution that solves most use cases: | ||
| - Users don't need additional infrastructure, such as server or object storage, to support larger `Results`. | ||
| - Existing `Tasks` will continue to function as they do now, while supporting larger `Results`, without any API changes. | ||
|
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||
| ### Performance | ||
|
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| Performance benchmarking with 20-30 `PipelineRuns`, each with 3 `TaskRuns` each with two `Steps`: | ||
| - Average `Pipeline` controller's CPU difference during `PipelineRun` execution: 1% | ||
| - Average `Pipeline` controller's Memory usage difference during `PipelineRun` execution: 0.2% | ||
| - Average `Pod` startup time (time to get to running state) difference: 3s per `TaskRun` | ||
|
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| In the experiment, we will continue to measure the startup overhead and explore ways to improve it. | ||
|
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| For further details, see the [performance metrics][performance]. | ||
|
|
||
| ### Security | ||
|
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| This approach requires that the `Pipeline` controller has access to `Pod` logs. The `Pipeline` controller already has | ||
| extensive permissions in the cluster, such as read access to `Secrets`. Expanding the access even further is a concern | ||
| for some users, but is also acceptable for some users given the advantages. We will document the extended permissions | ||
| so that users can make the right choice for their own use cases and requirements. | ||
|
|
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| ## Experiment Questions | ||
|
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| These are some questions we plan to answer in the experiment: | ||
|
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| - What impact does this change have on the startup and execution time of `TaskRuns` and `PipelineRuns`? Can we | ||
| improve the performance impact? | ||
|
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| - How reliable is using `Sidecar` logs to process `Results`? | ||
|
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| - How many users adopt this solution? How many are satisfied with it given the advantages and disadvantages? | ||
| We will conduct a user survey soon after the feature has been released. | ||
|
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| ## References | ||
|
|
||
| - [TEP-0086: Larger Results][tep-0086] | ||
| - [TEP-0086: Larger Results via Sidecar Logs][745] | ||
| - [Implementation Pull Request][poc] | ||
| - [Demonstration by Chitrang][demo] | ||
|
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||
| [docs]: https://tekton.dev/docs/pipelines/tasks/#emitting-results | ||
| [4012]: https://github.com/tektoncd/pipeline/issues/4012 | ||
| [4808]: https://github.com/tektoncd/pipeline/issues/4808 | ||
| [tep-0086]: ./0086-changing-the-way-result-parameters-are-stored.md | ||
| [tep-0086-alts]: ./0086-changing-the-way-result-parameters-are-stored.md#alternatives | ||
| [crd-size]: https://github.com/kubernetes/kubernetes/issues/82292 | ||
| [tep-0100]: ./0100-embedded-taskruns-and-runs-status-in-pipelineruns.md | ||
| [demo]: https://drive.google.com/file/d/1NrWudE_XBqweomiY24DP2Txnl1yN0yD9/view | ||
| [poc]: https://github.com/tektoncd/pipeline/pull/5695 | ||
| [performance]: https://github.com/tektoncd/community/pull/745#issuecomment-1206668381 | ||
| [745]: https://github.com/tektoncd/community/pull/745 |
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