How Search Works in Monorail


Our goal for Monorail‘s search design is to provide a fast issue search implementation for Monorail that can scale well past one million issues and give results in about two seconds. Monorail supports a wide range of query terms, so we cannot simply predefine indexes for all possible queries. Monorail also needs to be scalable in the number of requests to withstand DoS attacks, ill-behaved API clients, and normal traffic spikes. A key requirement of Monorail’s search is to give exact result counts even for fairly large result sets (up to 100,000 issues).


From 2005 to 2016, we tracked issues on, which stored issues in Bigtable and indexed them with Mustang ST (a structured search enhancement to Google's web search). This implementation suffered from highly complex queries and occasional outages. It relied on caching to serve popular queries and could suffer a “stampede” effect when the cache needed to be refilled after invalidations.

When the Monorail SQL database was being designed in 2011, Google Cloud SQL was much slower than it is today. Some key factors made a non-sharded design unacceptable:

  • The SQL database took too long to execute a query. Basically, the time taken is proportional to the number of Issue table rows considered. While indexes are used for many of Monorail's other queries, the issue search query is essentially a table scan in many cases. The question is how much of the table is scanned.

  • Getting SQL result rows into python was slow. The protocol between the database and app was inefficient, prompting some significant work-arounds that were eventually phased out. And, constructing a large number of ProtoRPC internal business objects was slow. Both steps were CPU intensive. Being CPU-bound produced a poor user experience because the amount of CPU time given to a GAE app is unpredictable, leading to frustrating latency on queries that seemed fine previously.

Overview of the approach

The design of our search implementation basically addresses these challenges point-by-point:

Because there is no one index that can narrow down the number of table rows considered for all possible user issue queries, we sharded the database so that each table scan is limited to one shard. For example, with 10 shards, we can use 10 database instances in parallel, each scanning only 1/10 of the table rows that would otherwise be scanned. This saves time in retrieving rows. Using 10 DB instances also increases the total amount of RAM available to those instances which increases their internal cache hit ratio and allows them to do more sorting in RAM rather than using slower disk-based methods.

Because constructing ProtoRPC objects was slow, we implemented RAM caches and used memcache to reduce the number of issues that need to be loaded from the DB and constructed in python for any individual user request. Using RAM caches means that we can serve traffic spikes for popular issues and queries well, but it also required us to implement a distributed cache invalidation feature.

Sharding queries at the DB level naturally led to sharding requests across multiple besearch instances in python. Using 10 besearch instances gives 10x the amount of CPU time available for constructing ProtoRPC objects. Of course, sharding means that we needed to implement a merge sort to produce an overall result list that is sorted.

Another aspect of our approach is that we reduce the work needed in the main SQL query as much as possible by mapping user-oriented terms to internal ID integers in python code before sending the query to SQL. This mapping could be done via JOIN clauses in the query, but the information needed for these mappings rarely changes and can be cached effectively in python RAM.

Detailed design: Architecture

The parts of Monorail's architecture relevant to search consists of:

  • The default GAE service that handles incoming requests from users, makes sharded queries to the besearch service, integrates the results, and responds to the user.

  • The besearch GAE service that handles sharded search requests from the default module and communicates with a DB instance. The besearch service handles two kinds of requests: search requests which generate results that are not permission-checked so that they can be shared among all users, and nonviewable requests that do permission checks in bulk for a given user.

  • A primary DB instance and 10 replicas. The database has an Invalidate table used for distributed invalidation. And, issue-related tables include a shard column that allows us to define a DB index that includes the shard ID. The worst (least specific) key used by our issue query is typically (shard, status_id) when searching for open issues and (shard, project_id) when searching all issues in a project.

  • There are RAM caches in the default and besearch service instances, and we use memcache for both search result sets and for business objects (e.g., projects, configs, issues, and users).

  • Monorail uses the GAE full-text search index library for full-text terms in the user query. These terms are processed before the query is sent to the SQL database. The slowness of GAE full-text search and the lack of integration between full-text terms and structured terms is a weakness of the current design.

Detailed design: Key algorithms

Query parsing

To convert the user's query string into an SQL statement, FrontendSearchPipeline first parses parentheses and OR statemeents, splitting up a query into separate subqueries that can be retrieved from the cache or sent to different backend shards.

The generated subqueries should collectively output the same set of search results as the initial query, but without using ORs or parentheses in their syntax. An example is that the query 'A (B OR C)' would be split into the subqueries ['A B', 'A C'].

Then, each besearch shard parses the subquery it was assigned using the helpers in search/query2ast. We first parse the into query terms using regular expressions. Then, we build an abstract syntax tree (AST). Then, we simplify that AST by doing cacheable lookups in python. Then, we convert the simplified AST into a set of LEFT JOIN, WHERE, and ORDER BY clauses.

It is possible for a query to fail to parse and raise an exception before the query is executed.

Result set representations

We represent issue search results as lists of global issue ID numbers (IIDs). We represent nonviewable issues as sets of IIDs.

To apply permission checks to search results, we simply use set membership: any issue IID that is in the nonviewable set for the current user is excluded from the allowed results.

Sharded query execution

To manage sharded requests to besearch backends, the FrontendSearchPipeline class does the following steps:

  1. The constructor checks the user query and can determine an error message to display to the user.

  2. SearchForIIDs() calls _StartBackendSearch() which determines the set of shards that need to be queried, checks memcache for known results and calls backends to provide any missing results. _StartBackendSearch() returns a list of rpc_tuples, which SearchForIIDs() waits on. Each rpc_tuple has a callback that contains some retry logic. Sharded nonviewable IIDs are also determined. For each shard, the allowed IIDs for the current user are computed by removing nonviewable IIDs from the list of result IIDs.

  3. MergeAndSortIssues() merges the sharded results into an overall result list of allowed IIDs by calling _NarrowFilteredIIDs() and _SortIssues(). An important aspect of this step is that only a subset of issues are retrieved. _NarrowFilteredIIDs() fetches a small set of sample issues and uses the existing sorted order of IIDs in each shard to narrow down the set of issues that could be displayed on the current pagination page. Once that subset is determined, _SortIssues() calls methods in framework/ to do the actual sorting.

Issue position in list

Monorail's flipper feature also uses the FrontendSearchPipeline class, but calls DetermineIssuePosition() rather than MergeAndSortIssues(). DetermineIssuePosition() also retrieves only a subset of the issues in the allowed IIDs list. For each shard, it uses a sample of a few issues to determine the sub-range of issues that must be retrieved, and then sorts those with the current issue to determine the number of issues in that shard that would precede the currently viewed issue. The position of the current issue in the overall result list is the sum of the counts of preceding issues in each shard. Candidates for the next and previous issues are also identified on a per-shard basis, and then the overall next and previous issues are determined.

Memcache keys and invalidation

We cache search results keyed by query and shard, regardless of the user or their permissions. This allows the app to reuse cached results for different users. When issues are edited, we only need to invalidate the shard that that issue belongs to.

The key format for search results in memcache is memcache_key_prefix, subquery, sd_str, sid, where:

  • memcache_key_prefix is a list of project IDs or all
  • subquery is the user query (or one OR-clause of it)
  • sd_str is the sort directive
  • sid is the shard ID number

If it were not for cross-project search, we would simply cache when we do a search and then invalidate when an issue is modified. But, with cross-project search we don‘t know all the memcache entries that would need to be invalidated. So, instead, we write the search result cache entries and then an initial modified_ts value for each project if it was not already there. And, when we update an issue we write a new modified_ts entry for that issue’s project shard. That entry implicitly invalidates all search result cache entries that were written earlier because they are now stale. When reading from the cache, we ignore any cache entry that corresponds to a project with modified_ts after the cached search result timestamp, because it is stale.

We cache nonviewable IID sets keyed by user ID, project ID, and shard ID, regardless of query. We only need to invalidate cached nonviewable IDs when a user role is granted or revoked, when an issue restriction label is changed, or a new restricted issue is created.

Detailed design: Code walk-throughs

Issue search

  1. The user makes a request to an issue list page. For the EZT issue list page, the usual request handling is done, including a call to IssueList#GatherPageData(). For, the web components list page or an API client, the ListIssues() API handler is called.

  2. One of those request handlers calls work_env.ListIssues() which constructs a FrontendSearchPipeline and works through the entire process to generate the list of issues to be returned for the current pagination page. The pipeline object is returned.

The FrontendSearchPipeline process steps are:

  1. A FrontendSearchPipeline object is created to manage the search process. It immediately parses some simple information from the request and initializes variables.

  2. WorkEnv calls SearchForIIDs() on the FrontendSearchPipeline. It loops over the shards and:

  • It checks memcache to see if that (query, sort, shard_id) is cached and the cache is still valid. If found, these can be used as unfiltered IIDs.

  • If not cached, it kicks off a request to one of the GAE besearch backend instances to get fresh unfiltered IIDs. Each backend translates the user query into an SQL query and executes it on one of the SQL replicas. Each backend stores a list of unfiltered IIDs in memcache.

  • In parallel, unviewable IIDs for the current user are looked up in the cache and possibly requested from the besearch backends.

  • Within each shard, unviewable IIDs are removed from the unfiltered IIDs to give sharded lists of filtered IIDs.

  • Sharded lists of filtered IIDs are combined into an overall result that has only the issues needed for the current pagination page. This step involves retrieving sample issues and a few distinct sorting steps.

  • Backend calls are made with the X-AppEngine-FailFast: Yes header set, which means that if the selected backend is already busy, the request immediately fails so that it can be retried on another backend that might not be busy. If there is an error or timeout during any backend call, a second attempt is made without the FailFast header. If that fails, that failure results in an error message saying that some backends did not respond.

Issue flipper

For the issue detail page, we do not need to completely sort and paginate the issues. Instead, we only need the count of allowed issues, the position of the current issue in the hypothetically sorted list, and the IDs of the previous and next issues, if any, which we call the “flipper” feature.

As of December 2019, the flipper does not use the pRPC API yet. Instead, it uses an older JSON servlet implementation. When it is implemented in the pRPC API, only the first few steps listed below will change.

The steps for the flipper are:

  1. The web components version of the issue detail page makes an XHR request to the flipper servlet with the search query and the current issue ref in query string parameters.

  2. The FlipperIndex servlet decides if a flipper should be shown, and whether the request is being made in the context of an issue search or a hotlist.

  3. It calls work_env.FindIssuePositionInSearch() to get the position of the current issue, previous and next issues, and the total count of allowed search results.

  4. Instead of calling the pipeline's MergeAndSortIssues(), the method DetermineIssuePosition() is called. It retrieves only a small fraction of the issues in each shard and determines the overall position of the current issue and the IID of the preceding and following issues in the sorted list.

We also have special servlets that redirect the user to the previous or next issues given a current issue and a query. These allow for faster navigation when the user clicks these links or uses the j or k keystrokes before the flipper has fully loaded.


To power the burndown charts feature, every issue create and update operation writes a new row to the IssueSnapshot table. When a user visits a chart page, the search pipeline runs a SELECT COUNT query on the IssueSnapshot table, instead of what it would normally do, running a SELECT query on the Issue table.

Any given issue will have many snapshots over time. The way we keep track of the most recent snapshots are with the columns IssueSnapshot.period_start and IssueSnapshot.period_end.

If you imagine a Monorail instance with only one issue, each time we add a new snapshot to the table, we update the previous snapshot‘s period_end and the new snapshot’s period_start to be the current unix time. This means that for a given range (period_start, period_end), there is only one snapshot that applies. The most recent snapshot always has its period_end set to MAX_INT.

Snapshot ID:  1         2         3                 MAX_INT

Unix time:
1560000004                        +-----------------+
1560000003              +---------+
1560000002    +---------+

Detailed design: Source code locations

  • framework/ Sorting of issues in RAM. See sorting design doc.

  • search/ Where searches are processed first. Sequences events for hitting sharded backends. Does set logic to remove nonviewable IIDs from the current user's search results. MergeAndSortIssues() combines search results from each shard into a unified result. Also, DetermineIssuePosition() function calculates the position of the current issue in a search result without merging the entire search result.

  • search/ Sequence of events to search for matching issues and at-risk issues, caching of unfiltered results, and calling code for permissions filtering. Also, calls ast2select and ast2sort to build the query, and combine SQL results with full-text results.

  • search/ Small backend servlet that handles the request for one shard from the frontend, uses a backendsearchpipeline instance, returns the results to the frontend as JSON including an unfiltered_iids list of global issue IDs. As a side-effect, each parallel execution of this servlet loads some of the issues that the frontend will very likely need and pre-caches them in memcache.

  • search/ Small backend servlet that finds issues in one shard of a project that the given user cannot view. This is not specific to the user's current query. It puts that result into memcache, and returns those IDs as JSON to the frontend.

  • search/ Utility functions used by both frontend and backend parts of the search process.

  • tracker/ Has a dictionary of key functions used when sorting issues in RAM.

  • services/ RunIssueQuery() runs an SQL query on a specific DB shard.

  • search/ parses the user’s query into an AST (abstract syntax tree).

  • search/ Simplifies the AST by doing some lookups in python for info that could be cached in RAM.

  • search/ Converts the AST into SQL clauses.

  • search/ Converts sort directives into SQL ORDER BY clauses.