@ -159,10 +159,35 @@ if any write failed to one of the replicas, multiple differing chunk objects
will be created in the backing store. See [Querier](#querier) for how data is
deduplicated.
The ingesters validate timestamps for each log line received maintains a
strict ordering. See the [Loki
Overview](../overview#timestamp-ordering) for detailed documentation on
the rules of timestamp order.
#### Timestamp Ordering
The ingester validates that ingested log lines are not out of order. When an
ingester receives a log line that doesn't follow the expected order, the line
is rejected and an error is returned to the user.
The ingester validates that ingested log lines are received in
timestamp-ascending order (i.e., each log has a timestamp that occurs at a later
time than the log before it). When the ingester receives a log that does not
follow this order, the log line is rejected and an error is returned.
Logs from each unique set of labels are built up into "chunks" in memory and
then flushed to the backing storage backend.
If an ingester process crashes or exits abruptly, all the data that has not yet
been flushed could be lost. Loki is usually configured with a [Write Ahead Log](../operations/storage/wal) which can be _replayed_ on restart as well as with a `replication_factor` (usually 3) of each log to mitigate this risk.
In general, all lines pushed to Loki for a given stream (unique combination of
labels) must have a newer timestamp than the line received before it. There are,
however, two cases for handling logs for the same stream with identical
nanosecond timestamps:
1. If the incoming line exactly matches the previously received line (matching
both the previous timestamp and log text), the incoming line will be treated
as an exact duplicate and ignored.
2. If the incoming line has the same timestamp as the previous line but
different content, the log line is accepted. This means it is possible to
have two different log lines for the same timestamp.
#### Handoff - Deprecated in favor of the [WAL](../operations/storage/wal)
@ -179,6 +204,13 @@ set of tokens.
This process is used to avoid flushing all chunks when shutting down, which is a
slow process.
#### Filesystem Support
While ingesters do support writing to the filesystem through BoltDB, this only
works in single-process mode as [queriers](#querier) need access to the same
back-end store and BoltDB only allows one process to have a lock on the DB at a
given time.
### 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.
@ -212,7 +244,7 @@ Caching log (filter, regexp) queries are under active development.
### Querier
The **querier** service handles queries using the [LogQL](../logql/) query
language, fetching logs both from the ingesters and long-term storage.
language, fetching logs both from the ingesters and from 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
This document builds upon the information in the [Loki Architecture](./) page.
Distributors are stateless and communicate with ingesters via [gRPC](https://grpc.io). The quantity of distributors can be increased or decreased as needed.
## Where does it live?
The distributor is the first component on Loki's write path downstream from any gateways providing auth or load balancing. It's responsible for validating, preprocessing, and applying a subset of rate limiting to incoming data before sending it to the ingester component. It is important that a load balancer sits in front of the distributor in order to properly balance traffic to them.
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 user.
### Ingester
The **ingester** service is responsible for writing log data to long-term
storage backends (DynamoDB, S3, Cassandra, etc.).
The ingester validates that ingested log lines are not out of order. When an
ingester receives a log line that doesn't follow the expected order, the line
is rejected and an error is returned to the user. See the section on [Timestamp
ordering](#timestamp-ordering) for more information.
The ingester validates that ingested log lines are received in
timestamp-ascending order (i.e., each log has a timestamp that occurs at a later
time than the log before it). When the ingester receives a log that does not
follow this order, the log line is rejected and an error is returned.
Logs from each unique set of labels are built up into "chunks" in memory and
then flushed to the backing storage backend.
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.
#### Timestamp Ordering
In general, all lines pushed to Loki for a given stream (unique combination of
labels) must have a newer timestamp than the line received before it. There are,
however, two cases for handling logs for the same stream with identical
nanosecond timestamps:
1. If the incoming line exactly matches the previously received line (matching
both the previous timestamp and log text), the incoming line will be treated
as an exact duplicate and ignored.
2. If the incoming line has the same timestamp as the previous line but
different content, the log line is accepted. This means it is possible to
have two different log lines for the same timestamp.
#### 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.
This process is used to avoid flushing all chunks when shutting down, which is a
slow process.
#### Filesystem Support
While ingesters do support writing to the filesystem through BoltDB, this only
works in single-process mode as [queriers](#querier) need access to the same
back-end store and BoltDB only allows one process to have a lock on the DB at a
given time.
### Querier
The **querier** service handles the actual [LogQL](../logql/) evaluation of
logs stored in long-term storage.
It first tries to query all ingesters for in-memory data before falling back to
loading data from the backend store.
### Query frontend
The **query-frontend** service is an optional component in front of a pool of queriers. It's responsible for fairly scheduling requests between them, paralleling them when possible, and caching.
## 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:
- An index for the chunks. This index can be backed by
[DynamoDB from Amazon Web Services](https://aws.amazon.com/dynamodb),
[Bigtable from Google Cloud Platform](https://cloud.google.com/bigtable), or
[Apache Cassandra](https://cassandra.apache.org).
- A key-value (KV) store for the chunk data itself, which can be DynamoDB,
Bigtable, Cassandra again, or an object store such as
[Amazon * S3](https://aws.amazon.com/s3)
> 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](#ingester) and [querier](#querier).
The chunk store relies on a unified interface to the
"[NoSQL](https://en.wikipedia.org/wiki/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 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
Karen Miller
5 years ago
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