Persistence
Metarank supports two possible persistence modes for storing features:
Persistence mode is configured by the optional state
section in the configuration file. By default, if the section is not defined, Metarank uses memory persistence.
Memory persistence
Memory persistence is no persistence at all: the complete Metarank state is stored only in RAM, is ephemeral, and will be entirely lost on each service restart.
Nevertheless, memory persistence can be useful:
While testing Metarank locally in a standalone mode, as it has no external service dependencies.
As a staging env to validate configuration changes before going to production.
To configure memory persistence, use the type: memory
option:
state:
type: memory
Redis persistence
Metarank can use Redis 6+ as a persistence method. To enable it, use the following config file snippet:
state:
type: redis
host: localhost
port: 6379
format: binary # optional, default=binary, possible values: json, binary
cache: # optional
maxSize: 1024 # size of in-memory client-side cache for hot keys, optional, default=1024
ttl: 1h # how long should key-values should be cached, optional, default=1h
clientTracking: true # should we subscribe for CLIENT TRACKING invalidation events
pipeline: # optional
maxSize: 128 # batch write buffer size, optional, default=128
flushPeriod: 1s # buffer flush interval, optional, default=1s
enabled: true # toggle pipelining, optional, default=true
auth: # optional
user: <username> # optional when Redis ACL is disabled
password: <password> # required if Redis server is run with requirepass argument
tls: # optional, defaults to disabled
enabled: true # optional, defaults to false
ca: <path/to/ca.crt> # optional path to the CA used to generate the cert, defaults to the default keychain
verify: full # optional, default=full, possible values: full, ca, off
# full - verify both certificate and hostname
# ca - verify only certificate
# off - skip verification
timeout: # optional, defaults to 1s for all sub-timeouts
connect: 1s # optional, defaults to 1s
socket: 1s # optional, defaults to 1s
command: 1s # optional, defaults to 1s
db: # optional, defaults to [0,1,2,3]: which redis dbs to use for persistence
state: 0 # can be used to co-locate multiple metarank instances
values: 1 # on a single redis server
rankings: 2
models: 3
Redis persistence is sensitive to network latencies (as it needs to perform a couple of round-trips on each event), hence Metarank leverages a couple of Redis performance optimization strategies:
Pipelining: all write operations are batched together and sent all at once
Client-side caching: read cache for hot keys with server-assisted invalidation.
A note on optional cache & pipelining related settings:
Metarank has a separate cache per underlying feature type (like scalar/counter/map/etc, 10 total), so
cache.maxSize
is set per cache type, so keep in mind an implicit multiplication: default value1024
in reality means10240
.cache.ttl
defines expiration interval after last read, so hot features may be cached almost indefinitely. The problem of stale cache values is solved with server-assisted invalidation: Redis server sends a notification to Metarank when key value was changed by someone else.pipeline.maxSize
going above128
is usually giving no benefit on low latencies (e.g. when Redis server is located in the same datacenter/AZ)pipeline.flushPeriod
controls the level of "eventualness" in the overall eventual consistency. With values larger than10
seconds, a second Metarank instance may not see write buffered in a first instance.
Disk persistence
Metarank has also an experimental option of using disk persistence instead of Redis. The main drawback of such an approach is that the deployment becomes stateful and you need to maintain a disk persistence.
Metarank supports two disk backends for file-based persistence:
MapDB: uses a mmap-based storage for data, works well for smaller datasets.
RocksDB: uses an LSM-tree storage, suits for large datasets.
The file persistence configured in the following way:
state:
type: file
path: /path/to/dir # required
format: binary # optional, default=binary, possible values: json, binary
backend: # optional, default mapdb
type: rocksdb # required, values: rocksdb, mapdb
RocksDB options
RocksDB can be configured by defining the following values in the config file:
state:
type: file
path: /path/to/dir # required
backend: # optional, default mapdb
type: rocksdb
lruCacheSizeMb: 1024000000 # LRU cache size in bytes, optional, default 1Gb
blockSize: 8192 # Block size in bytes, optional, default 8kb
A rule of thumb defining these parameters:
higher LRU cache size leads to better read throughput at the cost of extra memory usage. If not sure, set it to 50% of your RAM.
blockSize defines a size of page RocksDB reads from disk. In a perfect world it should match your actual disk block size: For cloud-attached disks like AWS EBS it should be 16kb, for local drives 1-2kb.
MapDB options
MapDB can be configured in the following way:
state:
type: file
path: /path/to/dir # required
backend: # optional, default mapdb
type: mapdb
mmap: true # should MapDB use mmap or raw disk reads for data access? Optional, default true.
maxNodeSize: 16 # what is the node size for internal db index. Optional, default 16.
TLS Support
Metarank supports connecting to Redis using TLS for transport encryption, but there is no way to autodetect the type of connection.
To connect to a TLS-enabled Redis server with self-signed certificate, you need to specify the CA used to sign the certificate (for self-signed certs it will be the server certificate itself):
enabled: true
ca: /tls/key.crt
To connect to a TLS-enabled Redis server with a certificate generated with default CA (for example, AWS ElastiCache Redis), then you don't need to specify any custom CA:
enabled: true
In a case when you have cert trust issues connecting to a TLS-enabled redis, you can downgrade the verification level. Supported levels are:
full
- verify both certificate and hostnameca
- verify only the certificateoff
- skip verification, trust all
An example:
enabled: true
verify: off
Authentication
auth.user
and auth.password
can control the credentials used to connect to Redis. As hardcoding the credentials into the config file is not usually considered secure, you can supply the credentials from environment variables:
METARANK_REDIS_USER
- only needed when Redis ACL is enabled.METARANK_REDIS_PASSWORD
- the pre-shared password used to connect to the Redis instance.
Metarank's Helm chart has a placeholder for the env variables passed inside the container inside Kubernetes. Usage example:
env:
- name: METARANK_REDIS_PASSWORD
valueFrom:
secretKeyRef:
name: redis-secret
key: REDIS_PASSWORD
State encoding formats
Metarank Redis persistence supports json
and binary
encoding formats for data stored in Redis:
json
: focused on readability and debugging simplicity.binary
: low-overhead binary encoding format, with better performance and smaller memory footprint.
binary
format on typical datasets (like RankLens) is ~2x faster and takes ~4x less RAM. We recommend it for larger datasets, when memory usage and associated costs are an important factor.
Redis support limitations
Metarank requires Redis 6+ due to a lack of client-side caching support in 5.x
you can disable client caching altogether (for example, for managed Redis-compatible engines, like GCP Memorystore Redis) with
cache.maxSize: 0
.For GCP Memorystore Redis, you can also set
state.cache.clientTracking: false
to disable theCLIENT TRACKING
cache eviction support: GCP Memstore has client-side caching disabled even on 7.x Redis cluster.
Redis Cluster is not yet supported; see ticket 568 for the progress.
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