--- stage: enablement group: Tenant Scale description: 'Cells Stateless Router Proposal' --- This document is a work-in-progress and represents a very early state of the Cells design. Significant aspects are not documented, though we expect to add them in the future. This is one possible architecture for Cells, and we intend to contrast this with alternatives before deciding which approach to implement. This documentation will be kept even if we decide not to implement this so that we can document the reasons for not choosing this approach. # Proposal: Stateless Router We will decompose `gitlab_users`, `gitlab_routes` and `gitlab_admin` related tables so that they can be shared between all cells and allow any cell to authenticate a user and route requests to the correct cell. Cells may receive requests for the resources they don't own, but they know how to redirect back to the correct cell. The router is stateless and does not read from the `routes` database which means that all interactions with the database still happen from the Rails monolith. This architecture also supports regions by allowing for low traffic databases to be replicated across regions. Users are not directly exposed to the concept of Cells but instead they see different data dependent on their chosen "organization". [Organizations](glossary.md#organizations) will be a new model introduced to enforce isolation in the application and allow us to decide which request route to which cell, since an organization can only be on a single cell. ## Differences The main difference between this proposal and one [with buffering requests](proposal-stateless-router-with-buffering-requests.md) is that this proposal uses a pre-flight API request (`/api/v4/cells/learn`) to redirect the request body to the correct Cell. This means that each request is sent exactly once to be processed, but the URI is used to decode which Cell it should be directed. ## Summary in diagrams This shows how a user request routes via DNS to the nearest router and the router chooses a cell to send the request to. ```mermaid graph TD; user((User)); dns[DNS]; router_us(Router); router_eu(Router); cell_us0{Cell US0}; cell_us1{Cell US1}; cell_eu0{Cell EU0}; cell_eu1{Cell EU1}; user-->dns; dns-->router_us; dns-->router_eu; subgraph Europe router_eu-->cell_eu0; router_eu-->cell_eu1; end subgraph United States router_us-->cell_us0; router_us-->cell_us1; end ``` ### More detail This shows that the router can actually send requests to any cell. The user will get the closest router to them geographically. ```mermaid graph TD; user((User)); dns[DNS]; router_us(Router); router_eu(Router); cell_us0{Cell US0}; cell_us1{Cell US1}; cell_eu0{Cell EU0}; cell_eu1{Cell EU1}; user-->dns; dns-->router_us; dns-->router_eu; subgraph Europe router_eu-->cell_eu0; router_eu-->cell_eu1; end subgraph United States router_us-->cell_us0; router_us-->cell_us1; end router_eu-.->cell_us0; router_eu-.->cell_us1; router_us-.->cell_eu0; router_us-.->cell_eu1; ``` ### Even more detail This shows the databases. `gitlab_users` and `gitlab_routes` exist only in the US region but are replicated to other regions. Replication does not have an arrow because it's too hard to read the diagram. ```mermaid graph TD; user((User)); dns[DNS]; router_us(Router); router_eu(Router); cell_us0{Cell US0}; cell_us1{Cell US1}; cell_eu0{Cell EU0}; cell_eu1{Cell EU1}; db_gitlab_users[(gitlab_users Primary)]; db_gitlab_routes[(gitlab_routes Primary)]; db_gitlab_users_replica[(gitlab_users Replica)]; db_gitlab_routes_replica[(gitlab_routes Replica)]; db_cell_us0[(gitlab_main/gitlab_ci Cell US0)]; db_cell_us1[(gitlab_main/gitlab_ci Cell US1)]; db_cell_eu0[(gitlab_main/gitlab_ci Cell EU0)]; db_cell_eu1[(gitlab_main/gitlab_ci Cell EU1)]; user-->dns; dns-->router_us; dns-->router_eu; subgraph Europe router_eu-->cell_eu0; router_eu-->cell_eu1; cell_eu0-->db_cell_eu0; cell_eu0-->db_gitlab_users_replica; cell_eu0-->db_gitlab_routes_replica; cell_eu1-->db_gitlab_users_replica; cell_eu1-->db_gitlab_routes_replica; cell_eu1-->db_cell_eu1; end subgraph United States router_us-->cell_us0; router_us-->cell_us1; cell_us0-->db_cell_us0; cell_us0-->db_gitlab_users; cell_us0-->db_gitlab_routes; cell_us1-->db_gitlab_users; cell_us1-->db_gitlab_routes; cell_us1-->db_cell_us1; end router_eu-.->cell_us0; router_eu-.->cell_us1; router_us-.->cell_eu0; router_us-.->cell_eu1; ``` ## Summary of changes 1. Tables related to User data (including profile settings, authentication credentials, personal access tokens) are decomposed into a `gitlab_users` schema 1. The `routes` table is decomposed into `gitlab_routes` schema 1. The `application_settings` (and probably a few other instance level tables) are decomposed into `gitlab_admin` schema 1. A new column `routes.cell_id` is added to `routes` table 1. A new Router service exists to choose which cell to route a request to. 1. If a router receives a new request it will send `/api/v4/cells/learn?method=GET&path_info=/group-org/project` to learn which Cell can process it 1. A new concept will be introduced in GitLab called an organization 1. We require all existing endpoints to be routable by URI, or be fixed to a specific Cell for processing. This requires changing ambiguous endpoints like `/dashboard` to be scoped like `/organizations/my-organization/-/dashboard` 1. Endpoints like `/admin` would be routed always to the specific Cell, like `cell_0` 1. Each Cell can respond to `/api/v4/cells/learn` and classify each endpoint 1. Writes to `gitlab_users` and `gitlab_routes` are sent to a primary PostgreSQL server in our `US` region but reads can come from replicas in the same region. This will add latency for these writes but we expect they are infrequent relative to the rest of GitLab. ## Pre-flight request learning While processing a request the URI will be decoded and a pre-flight request will be sent for each non-cached endpoint. When asking for the endpoint GitLab Rails will return information about the routable path. GitLab Rails will decode `path_info` and match it to an existing endpoint and find a routable entity (like project). The router will treat this as short-lived cache information. 1. Prefix match: `/api/v4/cells/learn?method=GET&path_info=/gitlab-org/gitlab-test/-/issues` ```json { "path": "/gitlab-org/gitlab-test", "cell": "cell_0", "source": "routable" } ``` 1. Some endpoints might require an exact match: `/api/v4/cells/learn?method=GET&path_info=/-/profile` ```json { "path": "/-/profile", "cell": "cell_0", "source": "fixed", "exact": true } ``` ## Detailed explanation of default organization in the first iteration All users will get a new column `users.default_organization` which they can control in user settings. We will introduce a concept of the `GitLab.com Public` organization. This will be set as the default organization for all existing users. This organization will allow the user to see data from all namespaces in `Cell US0` (ie. our original GitLab.com instance). This behavior can be invisible to existing users such that they don't even get told when they are viewing a global page like `/dashboard` that it's even scoped to an organization. Any new users with a default organization other than `GitLab.com Public` will have a distinct user experience and will be fully aware that every page they load is only ever scoped to a single organization. These users can never load any global pages like `/dashboard` and will end up being redirected to `/organizations//-/dashboard`. This may also be the case for legacy APIs and such users may only ever be able to use APIs scoped to a organization. ## Detailed explanation of Admin Area settings We believe that maintaining and synchronizing Admin Area settings will be frustrating and painful so to avoid this we will decompose and share all Admin Area settings in the `gitlab_admin` schema. This should be safe (similar to other shared schemas) because these receive very little write traffic. In cases where different cells need different settings (eg. the Elasticsearch URL), we will either decide to use a templated format in the relevant `application_settings` row which allows it to be dynamic per cell. Alternatively if that proves difficult we'll introduce a new table called `per_cell_application_settings` and this will have 1 row per cell to allow setting different settings per cell. It will still be part of the `gitlab_admin` schema and shared which will allow us to centrally manage it and simplify keeping settings in sync for all cells. ## Pros 1. Router is stateless and can live in many regions. We use Anycast DNS to resolve to nearest region for the user. 1. Cells can receive requests for namespaces in the wrong cell and the user still gets the right response as well as caching at the router that ensures the next request is sent to the correct cell so the next request will go to the correct cell 1. The majority of the code still lives in `gitlab` rails codebase. The Router doesn't actually need to understand how GitLab URLs are composed. 1. Since the responsibility to read and write `gitlab_users`, `gitlab_routes` and `gitlab_admin` still lives in Rails it means minimal changes will be needed to the Rails application compared to extracting services that need to isolate the domain models and build new interfaces. 1. Compared to a separate routing service this allows the Rails application to encode more complex rules around how to map URLs to the correct cell and may work for some existing API endpoints. 1. All the new infrastructure (just a router) is optional and a single-cell self-managed installation does not even need to run the Router and there are no other new services. ## Cons 1. `gitlab_users`, `gitlab_routes` and `gitlab_admin` databases may need to be replicated across regions and writes need to go across regions. We need to do an analysis on write TPS for the relevant tables to determine if this is feasible. 1. Sharing access to the database from many different Cells means that they are all coupled at the Postgres schema level and this means changes to the database schema need to be done carefully in sync with the deployment of all Cells. This limits us to ensure that Cells are kept in closely similar versions compared to an architecture with shared services that have an API we control. 1. Although most data is stored in the right region there can be requests proxied from another region which may be an issue for certain types of compliance. 1. Data in `gitlab_users` and `gitlab_routes` databases must be replicated in all regions which may be an issue for certain types of compliance. 1. The router cache may need to be very large if we get a wide variety of URLs (ie. long tail). In such a case we may need to implement a 2nd level of caching in user cookies so their frequently accessed pages always go to the right cell the first time. 1. Having shared database access for `gitlab_users` and `gitlab_routes` from multiple cells is an unusual architecture decision compared to extracting services that are called from multiple cells. 1. It is very likely we won't be able to find cacheable elements of a GraphQL URL and often existing GraphQL endpoints are heavily dependent on ids that won't be in the `routes` table so cells won't necessarily know what cell has the data. As such we'll probably have to update our GraphQL calls to include an organization context in the path like `/api/organizations//graphql`. 1. This architecture implies that implemented endpoints can only access data that are readily accessible on a given Cell, but are unlikely to aggregate information from many Cells. 1. All unknown routes are sent to the latest deployment which we assume to be `Cell US0`. This is required as newly added endpoints will be only decodable by latest cell. Likely this is not a problem for the `/cells/learn` is it is lightweight to process and this should not cause a performance impact. ## Example database configuration Handling shared `gitlab_users`, `gitlab_routes` and `gitlab_admin` databases, while having dedicated `gitlab_main` and `gitlab_ci` databases should already be handled by the way we use `config/database.yml`. We should also, already be able to handle the dedicated EU replicas while having a single US primary for `gitlab_users` and `gitlab_routes`. Below is a snippet of part of the database configuration for the Cell architecture described above. **Cell US0**: ```yaml # config/database.yml production: main: host: postgres-main.cell-us0.primary.consul load_balancing: discovery: postgres-main.cell-us0.replicas.consul ci: host: postgres-ci.cell-us0.primary.consul load_balancing: discovery: postgres-ci.cell-us0.replicas.consul users: host: postgres-users-primary.consul load_balancing: discovery: postgres-users-replicas.us.consul routes: host: postgres-routes-primary.consul load_balancing: discovery: postgres-routes-replicas.us.consul admin: host: postgres-admin-primary.consul load_balancing: discovery: postgres-admin-replicas.us.consul ``` **Cell EU0**: ```yaml # config/database.yml production: main: host: postgres-main.cell-eu0.primary.consul load_balancing: discovery: postgres-main.cell-eu0.replicas.consul ci: host: postgres-ci.cell-eu0.primary.consul load_balancing: discovery: postgres-ci.cell-eu0.replicas.consul users: host: postgres-users-primary.consul load_balancing: discovery: postgres-users-replicas.eu.consul routes: host: postgres-routes-primary.consul load_balancing: discovery: postgres-routes-replicas.eu.consul admin: host: postgres-admin-primary.consul load_balancing: discovery: postgres-admin-replicas.eu.consul ``` ## Request flows 1. `gitlab-org` is a top level namespace and lives in `Cell US0` in the `GitLab.com Public` organization 1. `my-company` is a top level namespace and lives in `Cell EU0` in the `my-organization` organization ### Experience for paying user that is part of `my-organization` Such a user will have a default organization set to `/my-organization` and will be unable to load any global routes outside of this organization. They may load other projects/namespaces but their MR/Todo/Issue counts at the top of the page will not be correctly populated in the first iteration. The user will be aware of this limitation. #### Navigates to `/my-company/my-project` while logged in 1. User is in Europe so DNS resolves to the router in Europe 1. They request `/my-company/my-project` without the router cache, so the router chooses randomly `Cell EU1` 1. The `/cells/learn` is sent to `Cell EU1`, which responds that resource lives on `Cell EU0` 1. `Cell EU0` returns the correct response 1. The router now caches and remembers any request paths matching `/my-company/*` should go to `Cell EU0` ```mermaid sequenceDiagram participant user as User participant router_eu as Router EU participant cell_eu0 as Cell EU0 participant cell_eu1 as Cell EU1 user->>router_eu: GET /my-company/my-project router_eu->>cell_eu1: /api/v4/cells/learn?method=GET&path_info=/my-company/my-project cell_eu1->>router_eu: {path: "/my-company", cell: "cell_eu0", source: "routable"} router_eu->>cell_eu0: GET /my-company/my-project cell_eu0->>user:

My Project... ``` #### Navigates to `/my-company/my-project` while not logged in 1. User is in Europe so DNS resolves to the router in Europe 1. The router does not have `/my-company/*` cached yet so it chooses randomly `Cell EU1` 1. The `/cells/learn` is sent to `Cell EU1`, which responds that resource lives on `Cell EU0` 1. `Cell EU0` redirects them through a login flow 1. User requests `/users/sign_in`, uses random Cell to run `/cells/learn` 1. The `Cell EU1` responds with `cell_0` as a fixed route 1. User after login requests `/my-company/my-project` which is cached and stored in `Cell EU0` 1. `Cell EU0` returns the correct response ```mermaid sequenceDiagram participant user as User participant router_eu as Router EU participant cell_eu0 as Cell EU0 participant cell_eu1 as Cell EU1 user->>router_eu: GET /my-company/my-project router_eu->>cell_eu1: /api/v4/cells/learn?method=GET&path_info=/my-company/my-project cell_eu1->>router_eu: {path: "/my-company", cell: "cell_eu0", source: "routable"} router_eu->>cell_eu0: GET /my-company/my-project cell_eu0->>user: 302 /users/sign_in?redirect=/my-company/my-project user->>router_eu: GET /users/sign_in?redirect=/my-company/my-project router_eu->>cell_eu1: /api/v4/cells/learn?method=GET&path_info=/users/sign_in cell_eu1->>router_eu: {path: "/users", cell: "cell_eu0", source: "fixed"} router_eu->>cell_eu0: GET /users/sign_in?redirect=/my-company/my-project cell_eu0-->>user:

Sign in... user->>router_eu: POST /users/sign_in?redirect=/my-company/my-project router_eu->>cell_eu0: POST /users/sign_in?redirect=/my-company/my-project cell_eu0->>user: 302 /my-company/my-project user->>router_eu: GET /my-company/my-project router_eu->>cell_eu0: GET /my-company/my-project router_eu->>cell_eu0: GET /my-company/my-project cell_eu0->>user:

My Project... ``` #### Navigates to `/my-company/my-other-project` after last step 1. User is in Europe so DNS resolves to the router in Europe 1. The router cache now has `/my-company/* => Cell EU0`, so the router chooses `Cell EU0` 1. `Cell EU0` returns the correct response as well as the cache header again ```mermaid sequenceDiagram participant user as User participant router_eu as Router EU participant cell_eu0 as Cell EU0 participant cell_eu1 as Cell EU1 user->>router_eu: GET /my-company/my-project router_eu->>cell_eu0: GET /my-company/my-project cell_eu0->>user:

My Project... ``` #### Navigates to `/gitlab-org/gitlab` after last step 1. User is in Europe so DNS resolves to the router in Europe 1. The router has no cached value for this URL so randomly chooses `Cell EU0` 1. `Cell EU0` redirects the router to `Cell US0` 1. `Cell US0` returns the correct response as well as the cache header again ```mermaid sequenceDiagram participant user as User participant router_eu as Router EU participant cell_eu0 as Cell EU0 participant cell_us0 as Cell US0 user->>router_eu: GET /gitlab-org/gitlab router_eu->>cell_eu0: /api/v4/cells/learn?method=GET&path_info=/gitlab-org/gitlab cell_eu0->>router_eu: {path: "/gitlab-org", cell: "cell_us0", source: "routable"} router_eu->>cell_us0: GET /gitlab-org/gitlab cell_us0->>user:

GitLab.org... ``` In this case the user is not on their "default organization" so their TODO counter will not include their normal todos. We may choose to highlight this in the UI somewhere. A future iteration may be able to fetch that for them from their default organization. #### Navigates to `/` 1. User is in Europe so DNS resolves to the router in Europe 1. Router does not have a cache for `/` route (specifically rails never tells it to cache this route) 1. The Router choose `Cell EU0` randomly 1. The Rails application knows the users default organization is `/my-organization`, so it redirects the user to `/organizations/my-organization/-/dashboard` 1. The Router has a cached value for `/organizations/my-organization/*` so it then sends the request to `POD EU0` 1. `Cell EU0` serves up a new page `/organizations/my-organization/-/dashboard` which is the same dashboard view we have today but scoped to an organization clearly in the UI 1. The user is (optionally) presented with a message saying that data on this page is only from their default organization and that they can change their default organization if it's not right. ```mermaid sequenceDiagram participant user as User participant router_eu as Router EU participant cell_eu0 as Cell EU0 user->>router_eu: GET / router_eu->>cell_eu0: GET / cell_eu0->>user: 302 /organizations/my-organization/-/dashboard user->>router: GET /organizations/my-organization/-/dashboard router->>cell_eu0: GET /organizations/my-organization/-/dashboard cell_eu0->>user:

My Company Dashboard... X-Gitlab-Cell-Cache={path_prefix:/organizations/my-organization/} ``` #### Navigates to `/dashboard` As above, they will end up on `/organizations/my-organization/-/dashboard` as the rails application will already redirect `/` to the dashboard page. ### Navigates to `/not-my-company/not-my-project` while logged in (but they don't have access since this project/group is private) 1. User is in Europe so DNS resolves to the router in Europe 1. The router knows that `/not-my-company` lives in `Cell US1` so sends the request to this 1. The user does not have access so `Cell US1` returns 404 ```mermaid sequenceDiagram participant user as User participant router_eu as Router EU participant cell_us1 as Cell US1 user->>router_eu: GET /not-my-company/not-my-project router_eu->>cell_us1: GET /not-my-company/not-my-project cell_us1->>user: 404 ``` #### Creates a new top level namespace The user will be asked which organization they want the namespace to belong to. If they select `my-organization` then it will end up on the same cell as all other namespaces in `my-organization`. If they select nothing we default to `GitLab.com Public` and it is clear to the user that this is isolated from their existing organization such that they won't be able to see data from both on a single page. ### Experience for GitLab team member that is part of `/gitlab-org` Such a user is considered a legacy user and has their default organization set to `GitLab.com Public`. This is a "meta" organization that does not really exist but the Rails application knows to interpret this organization to mean that they are allowed to use legacy global functionality like `/dashboard` to see data across namespaces located on `Cell US0`. The rails backend also knows that the default cell to render any ambiguous routes like `/dashboard` is `Cell US0`. Lastly the user will be allowed to navigate to organizations on another cell like `/my-organization` but when they do the user will see a message indicating that some data may be missing (eg. the MRs/Issues/Todos) counts. #### Navigates to `/gitlab-org/gitlab` while not logged in 1. User is in the US so DNS resolves to the US router 1. The router knows that `/gitlab-org` lives in `Cell US0` so sends the request to this cell 1. `Cell US0` serves up the response ```mermaid sequenceDiagram participant user as User participant router_us as Router US participant cell_us0 as Cell US0 user->>router_us: GET /gitlab-org/gitlab router_us->>cell_us0: GET /gitlab-org/gitlab cell_us0->>user:

GitLab.org... ``` #### Navigates to `/` 1. User is in US so DNS resolves to the router in US 1. Router does not have a cache for `/` route (specifically rails never tells it to cache this route) 1. The Router chooses `Cell US1` randomly 1. The Rails application knows the users default organization is `GitLab.com Public`, so it redirects the user to `/dashboards` (only legacy users can see `/dashboard` global view) 1. Router does not have a cache for `/dashboard` route (specifically rails never tells it to cache this route) 1. The Router chooses `Cell US1` randomly 1. The Rails application knows the users default organization is `GitLab.com Public`, so it allows the user to load `/dashboards` (only legacy users can see `/dashboard` global view) and redirects to router the legacy cell which is `Cell US0` 1. `Cell US0` serves up the global view dashboard page `/dashboard` which is the same dashboard view we have today ```mermaid sequenceDiagram participant user as User participant router_us as Router US participant cell_us0 as Cell US0 participant cell_us1 as Cell US1 user->>router_us: GET / router_us->>cell_us1: GET / cell_us1->>user: 302 /dashboard user->>router_us: GET /dashboard router_us->>cell_us1: /api/v4/cells/learn?method=GET&path_info=/dashboard cell_us1->>router_us: {path: "/dashboard", cell: "cell_us0", source: "routable"} router_us->>cell_us0: GET /dashboard cell_us0->>user:

Dashboard... ``` #### Navigates to `/my-company/my-other-project` while logged in (but they don't have access since this project is private) They get a 404. ### Experience for non-authenticated users Flow is similar to logged in users except global routes like `/dashboard` will redirect to the login page as there is no default organization to choose from. ### A new customers signs up They will be asked if they are already part of an organization or if they'd like to create one. If they choose neither they end up no the default `GitLab.com Public` organization. ### An organization is moved from 1 cell to another TODO ### GraphQL/API requests which don't include the namespace in the URL TODO ### The autocomplete suggestion functionality in the search bar which remembers recent issues/MRs TODO ### Global search TODO ## Administrator ### Loads `/admin` page 1. The `/admin` is locked to `Cell US0` 1. Some endpoints of `/admin`, like Projects in Admin are scoped to a Cell and users needs to choose the correct one in a dropdown, which results in endpoint like `/admin/cells/cell_0/projects`. Admin Area settings in Postgres are all shared across all cells to avoid divergence but we still make it clear in the URL and UI which cell is serving the Admin Area page as there is dynamic data being generated from these pages and the operator may want to view a specific cell. ## More Technical Problems To Solve ### Replicating User Sessions Between All Cells Today user sessions live in Redis but each cell will have their own Redis instance. We already use a dedicated Redis instance for sessions so we could consider sharing this with all cells like we do with `gitlab_users` PostgreSQL database. But an important consideration will be latency as we would still want to mostly fetch sessions from the same region. An alternative might be that user sessions get moved to a JWT payload that encodes all the session data but this has downsides. For example, it is difficult to expire a user session, when their password changes or for other reasons, if the session lives in a JWT controlled by the user. ### How do we migrate between Cells Migrating data between cells will need to factor all data stores: 1. PostgreSQL 1. Redis Shared State 1. Gitaly 1. Elasticsearch ### Is it still possible to leak the existence of private groups via a timing attack? If you have router in EU, and you know that EU router by default redirects to EU located Cells, you know their latency (lets assume 10 ms). Now, if your request is bounced back and redirected to US which has different latency (lets assume that roundtrip will be around 60 ms) you can deduce that 404 was returned by US Cell and know that your 404 is in fact 403. We may defer this until we actually implement a cell in a different region. Such timing attacks are already theoretically possible with the way we do permission checks today but the timing difference is probably too small to be able to detect. One technique to mitigate this risk might be to have the router add a random delay to any request that returns 404 from a cell. ## Should runners be shared across all cells? We have 2 options and we should decide which is easier: 1. Decompose runner registration and queuing tables and share them across all cells. This may have implications for scalability, and we'd need to consider if this would include group/project runners as this may have scalability concerns as these are high traffic tables that would need to be shared. 1. Runners are registered per-cell and, we probably have a separate fleet of runners for every cell or just register the same runners to many cells which may have implications for queueing ## How do we guarantee unique ids across all cells for things that cannot conflict? This project assumes at least namespaces and projects have unique ids across all cells as many requests need to be routed based on their ID. Since those tables are across different databases then guaranteeing a unique ID will require a new solution. There are likely other tables where unique IDs are necessary and depending on how we resolve routing for GraphQL and other APIs and other design goals it may be determined that we want the primary key to be unique for all tables.