meta-memcache 0.9.5

Modern, pure python, memcache client with support for new meta commands.

meta-memcache-py

Modern, pure python, memcache client with support for new meta commands.

Usage:

Install:

pip install meta-memcache

Configure a pool:

from meta_memcache import (
    Key,
    ServerAddress,
    ShardedCachePool,
    connection_pool_factory_builder,
)

pool = ShardedCachePool.from_server_addresses(
    servers=[
        ServerAddress(host="1.1.1.1", port=11211),
        ServerAddress(host="2.2.2.2", port=11211),
        ServerAddress(host="3.3.3.3", port=11211),
    ],
    connection_pool_factory_fn=connection_pool_factory_builder(),
)

The design is very pluggable. Rather than supporting a lot of features, it relies on dependency injection to configure behavior.

The CachePools expects a connection_pool_factory_fn callback to build the internal connection pool. And the connection pool receives a function to create a new memcache connection.

While this is very flexible, it can be complex to initialize, so there is a default builder provided to tune the most frequent things:

def connection_pool_factory_builder(
    initial_pool_size: int = 1,
    max_pool_size: int = 3,
    mark_down_period_s: float = DEFAULT_MARK_DOWN_PERIOD_S,
    connection_timeout: float = 1,
    recv_timeout: float = 1,
    no_delay: bool = True,
    read_buffer_size: int = 4096,
) -> Callable[[ServerAddress], ConnectionPool]:

• initial_pool_size: How many connections open for each host in the pool
• max_pool_size: Maximum number of connections to keep open for each host in the pool. Note that if there are no connections available in the pool, the client will open a new connection always, instead of just blocking waiting for a free connection. If you see too many connection creations in the stats, you might need to increase this setting.
• mark_down_period_s: When a network failure is detected, the destination host is marked down, and requests will fail fast, instead of trying to reconnect causing clients to block. A single client request will be checking if the host is still down every mark_down_period_s, while the others fail fast.
• connection_timeout: Timeout to stablish initial connection, ideally should be < 1 s for memcache servers in local network.
• recv_timeout: Timeout of requests. Ideally should be < 1 s for memcache servers in local network.
• no_delay: Wether to configure socket with NO_DELAY. This library tries to send requests as a single write(), so enabling no_delay is a good idea.
• read_buffer_size: This client tries to minimize memory allocation by reading bytes from the socket into a reusable read buffer. If the memcache response size is < read_buffer_size no memory allocations happen for the network read. Note: Each connection will have its own read buffer, so you must find a good balance between memory usage and reducing memory allocations. Note: While reading from the socket has zero allocations, the values will be deserialized and those will have the expected memory allocations.

If you need to customize how the sockets are created (IPv6, add auth, unix sockets) you will need to implement your own connection_pool_factory_builder and override the socket_factory_fn.

Use the pool:

cache_pool.set(key=Key("bar"), value=1, ttl=1000)

Keys:

String or Key named tuple

On the high-level commands you can use either plain strings as keys or the more advanced Key object that allows extra features like custom routing and unicode keys.

Custom routing:

You can control the routing of the keys setting a custom routing_key:

Key("key:1:2", routing_key="key:1")

This is useful if you have several keys related with each other. You can use the same routing key, and they will be placed in the same server. This is handy for speed of retrieval (single request instead of fan-out) and/or consistency (all will be gone or present, since they are stored in the same server). Note this is also risky, if you place all keys of a user in the same server, and the server goes down, the user life will be miserable.

Unicode keys:

Unicode keys are supported, the keys will be hashed according to Meta commands binary encoded keys specification.

To use this, mark the key as unicode:

Key("🍺", unicode=True)

Large keys:

Large keys are also automatically supported and binary encoded as above. But don't use large keys :)

Design:

The code relies on dependency injection to allow to configure a lot of the aspects of the cache client. For example, instead of supporting a lot of features on how to connect, authenticate, etc, a socket_factory_fn is required and you can customize the socket creation to your needs. We provide some basic sane defaults, but you should not have a lot of issues to customize it for your needs.

Regarding cache client features, relies in inheritance to abstract different layers of responsibility, augment the capabilities while abstracting details out:

Low level meta commands:

The low-level class is BaseCachePool. Implements the connection pool handling as well as the memcache protocol, focusing on the new Memcache MetaCommands: meta get, meta set, meta delete and meta arithmetic. They implement the full set of flags, and features, but are very low level.

    def meta_multiget(
        self,
        keys: List[Key],
        flags: Optional[Set[Flag]] = None,
        int_flags: Optional[Dict[IntFlag, int]] = None,
        token_flags: Optional[Dict[TokenFlag, bytes]] = None,
    ) -> Dict[Key, ReadResponse]:

    def meta_get(
        self,
        key: Key,
        flags: Optional[Set[Flag]] = None,
        int_flags: Optional[Dict[IntFlag, int]] = None,
        token_flags: Optional[Dict[TokenFlag, bytes]] = None,
    ) -> ReadResponse:

    def meta_set(
        self,
        key: Key,
        value: Any,
        ttl: int,
        flags: Optional[Set[Flag]] = None,
        int_flags: Optional[Dict[IntFlag, int]] = None,
        token_flags: Optional[Dict[TokenFlag, bytes]] = None,
    ) -> WriteResponse:

    def meta_delete(
        self,
        key: Key,
        flags: Optional[Set[Flag]] = None,
        int_flags: Optional[Dict[IntFlag, int]] = None,
        token_flags: Optional[Dict[TokenFlag, bytes]] = None,
    ) -> WriteResponse:

    def meta_arithmetic(
        self,
        key: Key,
        flags: Optional[Set[Flag]] = None,
        int_flags: Optional[Dict[IntFlag, int]] = None,
        token_flags: Optional[Dict[TokenFlag, bytes]] = None,
    ) -> WriteResponse:

You won't use this api unless you are implementing some custom high-level command. See below for the usual memcache api.

High level commands:

The CachePool augments the low-level class and implements the more high-level memcache operations (get, set, touch, cas...), plus the memcached's new MetaCommands anti-dogpiling techniques for high qps caching needs: Atomic Stampeding control, Herd Handling, Early Recache, Serve Stale, No Reply, Probabilistic Hot Cache, Hot Key Cache Invalidation...

    def set(
        self,
        key: Union[Key, str],
        value: Any,
        ttl: int,
        no_reply: bool = False,
        cas_token: Optional[int] = None,
        stale_policy: Optional[StalePolicy] = None,
        set_mode: SetMode = SetMode.SET,  # Other are ADD, REPLACE, APPEND...
    ) -> bool:

    def delete(
        self,
        key: Union[Key, str],
        cas_token: Optional[int] = None,
        no_reply: bool = False,
        stale_policy: Optional[StalePolicy] = None,
    ) -> bool:

    def touch(
        self,
        key: Union[Key, str],
        ttl: int,
        no_reply: bool = False,
    ) -> bool:

    def get_or_lease(
        self,
        key: Union[Key, str],
        lease_policy: LeasePolicy,
        touch_ttl: Optional[int] = None,
        recache_policy: Optional[RecachePolicy] = None,
    ) -> Optional[Any]:

    def get_or_lease_cas(
        self,
        key: Union[Key, str],
        lease_policy: LeasePolicy,
        touch_ttl: Optional[int] = None,
        recache_policy: Optional[RecachePolicy] = None,
    ) -> Tuple[Optional[Any], Optional[int]]:

    def get(
        self,
        key: Union[Key, str],
        touch_ttl: Optional[int] = None,
        recache_policy: Optional[RecachePolicy] = None,
    ) -> Optional[Any]:

    def multi_get(
        self,
        keys: List[Union[Key, str]],
        touch_ttl: Optional[int] = None,
        recache_policy: Optional[RecachePolicy] = None,
    ) -> Dict[Key, Optional[Any]]:

    def get_cas(
        self,
        key: Union[Key, str],
        touch_ttl: Optional[int] = None,
        recache_policy: Optional[RecachePolicy] = None,
    ) -> Tuple[Optional[Any], Optional[int]]:

    def get_typed(
        self,
        key: Union[Key, str],
        cls: Type[T],
        touch_ttl: Optional[int] = None,
        recache_policy: Optional[RecachePolicy] = None,
        error_on_type_mismatch: bool = False,
    ) -> Optional[T]:

    def get_cas_typed(
        self,
        key: Union[Key, str],
        cls: Type[T],
        touch_ttl: Optional[int] = None,
        recache_policy: Optional[RecachePolicy] = None,
        error_on_type_mismatch: bool = False,
    ) -> Tuple[Optional[T], Optional[int]]:

    def delta(
        self,
        key: Union[Key, str],
        delta: int,
        refresh_ttl: Optional[int] = None,
        no_reply: bool = False,
        cas_token: Optional[int] = None,
    ) -> bool:

    def delta_initialize(
        self,
        key: Union[Key, str],
        delta: int,
        initial_value: int,
        initial_ttl: int,
        refresh_ttl: Optional[int] = None,
        no_reply: bool = False,
        cas_token: Optional[int] = None,
    ) -> bool:

    def delta_and_get(
        self,
        key: Union[Key, str],
        delta: int,
        refresh_ttl: Optional[int] = None,
        cas_token: Optional[int] = None,
    ) -> Optional[int]:

    def delta_initialize_and_get(
        self,
        key: Union[Key, str],
        delta: int,
        initial_value: int,
        initial_ttl: int,
        refresh_ttl: Optional[int] = None,
        cas_token: Optional[int] = None,
    ) -> Optional[int]:

Anti-dogpiling techniques

Some commands receive RecachePolicy, StalePolicy and LeasePolicy for the advanced anti-dogpiling control needed in high-qps environments:

class RecachePolicy(NamedTuple):
    """
    This controls the recache herd control behavior
    If recache ttl is indicated, when remaining ttl is < given value
    one of the clients will win, return a miss and will populate the
    value, while the other clients will loose and continue to use the
    stale value.
    """

    ttl: int = 30


class LeasePolicy(NamedTuple):
    """
    This controls the lease or miss herd control behavior
    If miss lease retries > 0, on misses a lease will be created. The
    winner will get a Miss and will continue to populate the cache,
    while the others are BLOCKED! Use with caution! You can define
    how many times and how often clients will retry to get the
    value. After the retries are expired, clients will get a Miss
    if they weren't able to get the value.
    """

    ttl: int = 30
    miss_retries: int = 3
    miss_retry_wait: float = 1.0
    wait_backoff_factor: float = 1.2
    miss_max_retry_wait: float = 5.0


class StalePolicy(NamedTuple):
    """
    This controls the stale herd control behavior
    * Deletions can mark items stale instead of deleting them
    * Stale items automatically do recache control, one client
      will get the miss, others will receive the stale value
      until the winner refreshes the value in the cache.
    * cas mismatches (due to race / further invalidation) can
      store the value as stale instead of failing
    """

    mark_stale_on_deletion_ttl: int = 0  # 0 means disabled
    mark_stale_on_cas_mismatch: bool = False

Notes:

• Recache/Stale policies are typically used together. Make sure all your reads for a given key share the same recache policy to avoid unexpected behaviors.
• Leases are for a more traditional, more consistent model, where other clients will block instead of getting a stale value.

Pool level features:

Finally in cache_pools.py a few classes implement the pool-level semantics:

• ShardedCachePool: implements a consistent hashing cache pool using uhashring's HashRing.
• ShardedWithGutterCachePool: implements a sharded cache pool like above, but with a 'gutter pool' (See Scaling Memcache at Facebook), so when a server of the primary pool is down, requests are sent to the 'gutter' pool, with TTLs overriden and lowered on the fly, so they provide some level of caching instead of hitting the backend for each request.

These pools also provide an option to register a callback for write failure events. This might be useful if you are serious about cache consistency. If you have transient network issues, some writes might fail, and if the server comes back without being restarted or the cache flushed, the data will be stale. The events allows for failed writes to be collected and logged. Affected keys can then be invalidated later and eventual cache consistency guaranteed.

It should be trivial to implement your own cache pool if you need custom sharding, shadowing, pools that support live migrations, etc. Feel free to contribute!

Write failure tracking

When a write failure occures with a SET or DELETE opperation occures then the pool.on_write_failure event handler will be triggered. Consumers subscribing to this handler will receive the key that failed. The following is an example on how to subscribe to these events:

from meta_memcache import CachePool, Key

class SomeConsumer(object):
    def __init__(self, pool: CachePool):
        self.pool = pool
        self.pool.on_write_failures += self.on_write_failure_handler

    def call_before_dereferencing(self):
        self.pool.on_write_failures -= self.on_write_failure_handler

    def on_write_failure_handler(self, key: Key) -> None:
        # Handle the failures here
        pass
        

Stats:

The cache pools offer a get_counters() that return information about the state of the servers and their connection pools:

    def get_counters(self) -> Dict[ServerAddress, PoolCounters]:

The counters are:

class PoolCounters(NamedTuple):
    # Available connections in the pool, ready to use
    available: int
    # The # of connections active, currently in use, out of the pool
    active: int
    # Current stablished connections (available + active)
    stablished: int
    # Total # of connections created. If this keeps growing
    # might mean the pool size is too small and we are
    # constantly needing to create new connections:
    total_created: int
    # Total # of connection or socket errors
    total_errors: int