Loki’s Architecture

Architecture

This document will expand on the information detailed in the Loki Overview.

Multi Tenancy

All data - both in memory and in long-term storage - is partitioned by a tenant ID, pulled from the X-Scope-OrgID HTTP header in the request when Loki is running in multi-tenant mode. When Loki is not in multi-tenant mode, the header is ignored and the tenant ID is set to “fake”, which will appear in the index and in stored chunks.

Modes of Operation

modes_diagram

Loki has a set of components (defined below in Components) which are internally referred to as modules. Each component spawns a gRPC server for internal traffic and an HTTP/1 server for external API requests. All components come with an HTTP/1 server, but most only expose readiness, health, and metrics endpoints.

Which component Loki runs is determined by either the -target flag at the command line or the target: <string> section in Loki’s config file. When the value of target is all, Loki will run all of its components in a single process. This is referred to as “single process”, “single binary”, or monolithic mode. Monolithic mode is the default deployment of Loki when Loki is installed using Helm.

When target is not set to all (i.e., it is set to querier, ingester, query-frontend, or distributor), then Loki is said to be running in “horizontally scalable”, or microservices, mode.

Each component of Loki, such as the ingesters and distributors, communicate with one another over gRPC using the gRPC listen port defined in the Loki config. When running components in monolithic mode, this is still true: each component, although running in the same process, will connect to each other over the local network for inter-component communication.

Single process mode is ideally suited for local development, small workloads, and for evaluation purposes. Monolithic mode can be scaled with multiple processes with the following limitations:

  1. Local index and local storage cannot currently be used when running monolithic mode with more than one replica, as each replica must be able to access the same storage backend, and local storage is not safe for concurrent access.
  2. Individual components cannot be scaled independently, so it is not possible to have more read components than write components.

Components

Distributor

The distributor service is responsible for handling incoming streams by clients. It’s the first stop in the write path for log data. Once the distributor receives a set of streams, each stream is validated for correctness and to ensure that it is within the configured tenant (or global) limits. Valid chunks are then split into batches and sent to multiple ingesters in parallel.

Hashing

Distributors use consistent hashing in conjunction with a configurable replication factor to determine which instances of the ingester service should receive a given stream.

A stream is a set of logs associated to a tenant and a unique labelset. The stream is hashed using both the tenant ID and the labelset and then the hash is used to find the ingesters to send the stream to.

A hash ring stored in Consul is used to achieve consistent hashing; all ingesters register themselves into the hash ring with a set of tokens they own. Each token is a random unsigned 32-bit number. Along with a set of tokens, ingesters register their state into the hash ring. The state JOINING, and ACTIVE may all receive write requests, while ACTIVE and LEAVING ingesters may receive read requests. When doing a hash lookup, distributors only use tokens for ingesters who are in the appropriate state for the request.

To do the hash lookup, distributors find the smallest appropriate token whose value is larger than the hash of the stream. When the replication factor is larger than 1, the next subsequent tokens (clockwise in the ring) that belong to different ingesters will also be included in the result.

The effect of this hash set up is that each token that an ingester owns is responsible for a range of hashes. If there are three tokens with values 0, 25, and 50, then a hash of 3 would be given to the ingester that owns the token 25; the ingester owning token 25 is responsible for the hash range of 1-25.

Quorum consistency

Since all distributors share access to the same hash ring, write requests can be sent to any distributor.

To ensure consistent query results, Loki uses Dynamo-style quorum consistency on reads and writes. This means that the distributor will wait for a positive response of at least one half plus one of the ingesters to send the sample to before responding to the client that initiated the send.

Ingester

The ingester service is responsible for writing log data to long-term storage backends (DynamoDB, S3, Cassandra, etc.) on the write path and returning log data for in-memory queries on the read path.

Ingesters contain a lifecycler which manages the lifecycle of an ingester in the hash ring. Each ingester has a state of either PENDING, JOINING, ACTIVE, LEAVING, or UNHEALTHY:

  1. PENDING is an Ingester’s state when it is waiting for a handoff from another ingester that is LEAVING.

  2. JOINING is an Ingester’s state when it is currently inserting its tokens into the ring and initializing itself. It may receive write requests for tokens it owns.

  3. ACTIVE is an Ingester’s state when it is fully initialized. It may receive both write and read requests for tokens it owns.

  4. LEAVING is an Ingester’s state when it is shutting down. It may receive read requests for data it still has in memory.

  5. UNHEALTHY is an Ingester’s state when it has failed to heartbeat to Consul. UNHEALTHY is set by the distributor when it periodically checks the ring.

Each log stream that an ingester receives is built up into a set of many “chunks” in memory and flushed to the backing storage backend at a configurable interval.

Chunks are compressed and marked as read-only when:

  1. The current chunk has reached capacity (a configurable value).
  2. Too much time has passed without the current chunk being updated
  3. A flush occurs.

Whenever a chunk is compressed and marked as read-only, a writable chunk takes its place.

If an ingester process crashes or exits abruptly, all the data that has not yet been flushed will be lost. Loki is usually configured to replicate multiple replicas (usually 3) of each log to mitigate this risk.

When a flush occurs to a persistent storage provider, the chunk is hashed based on its tenant, labels, and contents. This means that multiple ingesters with the same copy of data will not write the same data to the backing store twice, but if any write failed to one of the replicas, multiple differing chunk objects will be created in the backing store. See Querier for how data is deduplicated.

The ingesters validate timestamps for each log line received maintains a strict ordering. See the Loki Overview for detailed documentation on the rules of timestamp order.

Handoff

By default, when an ingester is shutting down and tries to leave the hash ring, it will wait to see if a new ingester tries to enter before flushing and will try to initiate a handoff. The handoff will transfer all of the tokens and in-memory chunks owned by the leaving ingester to the new ingester.

Before joining the hash ring, ingesters will wait in PENDING state for a handoff to occur. After a configurable timeout, ingesters in the PENDING state that have not received a transfer will join the ring normally, inserting a new set of tokens.

This process is used to avoid flushing all chunks when shutting down, which is a slow process.

Query frontend

The query frontend is an optional service providing the querier’s API endpoints and can be used to accelerate the read path. When the query frontend is in place, incoming query requests should be directed to the query frontend instead of the queriers. The querier service will be still required within the cluster, in order to execute the actual queries.

The query frontend internally performs some query adjustments and holds queries in an internal queue. In this setup, queriers act as workers which pull jobs from the queue, execute them, and return them to the query-frontend for aggregation. Queriers need to be configured with the query frontend address (via the -querier.frontend-address CLI flag) in order to allow them to connect to the query frontends.

Query frontends are stateless. However, due to how the internal queue works, it’s recommended to run a few query frontend replicas to reap the benefit of fair scheduling. Two replicas should suffice in most cases.

Queueing

The query frontend queuing mechanism is used to:

  • Ensure that large queries, that could cause an out-of-memory (OOM) error in the querier, will be retried on failure. This allows administrators to under-provision memory for queries, or optimistically run more small queries in parallel, which helps to reduce the TCO.
  • Prevent multiple large requests from being convoyed on a single querier by distributing them across all queriers using a first-in/first-out queue (FIFO).
  • Prevent a single tenant from denial-of-service-ing (DOSing) other tenants by fairly scheduling queries between tenants.

Splitting

The query frontend splits larger queries into multiple smaller queries, executing these queries in parallel on downstream queriers and stitching the results back together again. This prevents large (multi-day, etc) queries from causing out of memory issues in a single querier and helps to execute them faster.

Caching

Metric Queries

The query frontend supports caching metric query results and reuses them on subsequent queries. If the cached results are incomplete, the query frontend calculates the required subqueries and executes them in parallel on downstream queriers. The query frontend can optionally align queries with their step parameter to improve the cacheability of the query results. The result cache is compatible with any loki caching backend (currently memcached, redis, and an in-memory cache).

Log Queries - Coming soon!

Caching log (filter, regexp) queries are under active development.

Querier

The querier service handles queries using the LogQL query language, fetching logs both from the ingesters and long-term storage.

Queriers query all ingesters for in-memory data before falling back to running the same query against the backend store. Because of the replication factor, it is possible that the querier may receive duplicate data. To resolve this, the querier internally deduplicates data that has the same nanosecond timestamp, label set, and log message.

Chunk Format

  -------------------------------------------------------------------
  |                               |                                 |
  |        MagicNumber(4b)        |           version(1b)           |
  |                               |                                 |
  -------------------------------------------------------------------
  |         block-1 bytes         |          checksum (4b)          |
  -------------------------------------------------------------------
  |         block-2 bytes         |          checksum (4b)          |
  -------------------------------------------------------------------
  |         block-n bytes         |          checksum (4b)          |
  -------------------------------------------------------------------
  |                        #blocks (uvarint)                        |
  -------------------------------------------------------------------
  | #entries(uvarint) | mint, maxt (varint) | offset, len (uvarint) |
  -------------------------------------------------------------------
  | #entries(uvarint) | mint, maxt (varint) | offset, len (uvarint) |
  -------------------------------------------------------------------
  | #entries(uvarint) | mint, maxt (varint) | offset, len (uvarint) |
  -------------------------------------------------------------------
  | #entries(uvarint) | mint, maxt (varint) | offset, len (uvarint) |
  -------------------------------------------------------------------
  |                      checksum(from #blocks)                     |
  -------------------------------------------------------------------
  |                    #blocks section byte offset                  |
  -------------------------------------------------------------------

mint and maxt describe the minimum and maximum Unix nanosecond timestamp, respectively.

Block Format

A block is comprised of a series of entries, each of which is an individual log line.

Note that the bytes of a block are stored compressed using Gzip. The following is their form when uncompressed:

  -------------------------------------------------------------------
  |    ts (varint)    |     len (uvarint)    |     log-1 bytes      |
  -------------------------------------------------------------------
  |    ts (varint)    |     len (uvarint)    |     log-2 bytes      |
  -------------------------------------------------------------------
  |    ts (varint)    |     len (uvarint)    |     log-3 bytes      |
  -------------------------------------------------------------------
  |    ts (varint)    |     len (uvarint)    |     log-n bytes      |
  -------------------------------------------------------------------

ts is the Unix nanosecond timestamp of the logs, while len is the length in bytes of the log entry.

Chunk Store

The chunk store is Loki’s long-term data store, designed to support interactive querying and sustained writing without the need for background maintenance tasks. It consists of:

Unlike the other core components of Loki, the chunk store is not a separate service, job, or process, but rather a library embedded in the two services that need to access Loki data: the ingester and querier.

The chunk store relies on a unified interface to the “NoSQL” stores (DynamoDB, Bigtable, and Cassandra) that can be used to back the chunk store index. This interface assumes that the index is a collection of entries keyed by:

  • A hash key. This is required for all reads and writes.
  • A range key. This is required for writes and can be omitted for reads, which can be queried by prefix or range.

The interface works somewhat differently across the supported databases:

  • DynamoDB supports range and hash keys natively. Index entries are thus modelled directly as DynamoDB entries, with the hash key as the distribution key and the range as the DynamoDB range key.
  • For Bigtable and Cassandra, index entries are modelled as individual column values. The hash key becomes the row key and the range key becomes the column key.

A set of schemas are used to map the matchers and label sets used on reads and writes to the chunk store into appropriate operations on the index. Schemas have been added as Loki has evolved, mainly in an attempt to better load balance writes and improve query performance.

Read Path

To summarize, the read path works as follows:

  1. The querier receives an HTTP/1 request for data.
  2. The querier passes the query to all ingesters for in-memory data.
  3. The ingesters receive the read request and return data matching the query, if any.
  4. The querier lazily loads data from the backing store and runs the query against it if no ingesters returned data.
  5. The querier iterates over all received data and deduplicates, returning a final set of data over the HTTP/1 connection.

Write Path

chunk_diagram

To summarize, the write path works as follows:

  1. The distributor receives an HTTP/1 request to store data for streams.
  2. Each stream is hashed using the hash ring.
  3. The distributor sends each stream to the appropriate ingesters and their replicas (based on the configured replication factor).
  4. Each ingester will create a chunk or append to an existing chunk for the stream’s data. A chunk is unique per tenant and per labelset.
  5. The distributor responds with a success code over the HTTP/1 connection.