Backends

Common

Common functionality for implementing backends.

exception ska_sdp_config.backend.common.ConfigCollision(path: str, message: str)

Exception generated if key to create already exists.

exception ska_sdp_config.backend.common.ConfigVanished(path: str, message: str)

Exception generated if key to update that does not exist.

ska_sdp_config.backend.common.depth_of_path(path: str) → int

Get the depth of a path, this is the number of “/” in it.

Returns:

the depth

Etcd3 backend

Etcd3 backend for SKA SDP configuration DB.

class ska_sdp_config.backend.etcd3.Etcd3Backend(host='127.0.0.1', port='2379', max_retries: int = 15, retry_time: float = 0.1, **kw_args)

Highly consistent database backend store.

See https://github.com/kragniz/python-etcd3

close() → None

Close the client connection.

create(path: str, value: str, lease: Lease | None = None) → None

Create a key and initialise it with the value.

Parameters:

• path – Path to create

• value – Value to set

• lease – Lease to associate

Raises:

ConfigCollision if the key already exists

delete(path: str, must_exist: bool = True, recursive: bool = False, prefix: bool = False, max_depth: int = 16)

Delete the given key or key range.

Parameters:

• path – path (prefix) of keys to remove

• must_exist – Fail if path does not exist?

• recursive – Delete children keys at lower levels recursively

• max_depth – Recursion limit

• prefix – Delete all keys at given level with prefix

get(path: str, revision: DbRevision | None = None) → tuple[str, DbRevision]

Get value of a key.

Parameters:

• path – Path of key to query

• revision – to get

Returns:

value and revision

lease(ttl: float = 10) → Lease

Generate a new lease.

Once entered, it can be associated with keys which will be kept alive until the end of the lease.

Note that this involves starting a daemon thread that will refresh the lease periodically (default seems to be TTL/4).

Parameters:

ttl – Time to live for lease

Returns:

lease object

list_keys(path: str, recurse: int | Iterable[int] = 0, revision: DbRevision | None = None, with_values: bool = False) → tuple[list[str], DbRevision]

List keys under given path.

Parameters:

• path – Prefix of keys to query. Append ‘/’ to list child paths.

• recurse – Maximum recursion level to query. If iterable, cover exactly the recursion levels specified.

• revision – Database revision for which to list

• with_values – Also return key values and mod revisions (i.e. sorted list of key-value-rev tuples)

Returns:

(sorted key list, DbRevision object)

property server_version: Version

Returns the version of the server

txn(max_retries: int = 64) → Iterable[Etcd3Transaction]

Create a new transaction.

Note that this uses an optimistic STM-style implementation, which cannot guarantee that a transaction runs through successfully. Therefore, this function returns an iterator, which loops until the transaction succeeds:

for txn in etcd3.txn():
    # ... transaction steps ...

Note that this will in most cases only execute one iteration. If you actually want to loop - for instance because you intend to wait for something to happen in the configuration - use watcher() instead.

Parameters:

max_retries – Maximum number of transaction loops

Returns:

Transaction iterator

update(path: str, value: str) → None

Update an existing key. Fails if the key does not exist.

Parameters:

• path – Path to update

• value – New value of key

Raises:

ConfigVanished if the key does not exist

watcher(timeout=None, txn_wrapper: Callable[['Etcd3Transaction'], object] = None, requery_progress: float = 0.2) → Iterable[Etcd3Watcher]

Create a new watcher.

Useful for waiting for changes in the configuration. See etcd3_watcher.Etcd3Watcher.

Parameters:

• timeout – Timeout for waiting. Watcher will loop after this time.

• txn_wrapper – Function to wrap transactions returned by the wrapper.

• requery_progress – How often we “refresh” the current database state for watcher transactions even without watcher notification (upper bound on how “stale” non-watched values retrieved in transactions can be)

Returns:

Watcher iterator

class ska_sdp_config.backend.etcd3.Etcd3Transaction(backend: Etcd3Backend, client: client, max_retries: int = 64)

A series of queries and updates to be executed atomically.

commit() → bool

Commit the transaction to the database.

This can fail, in which case the transaction must get reset and built again.

Returns:

Whether the commit succeeded

create(path: str, value: str, lease: Lease | None = None) → None

Create a key and initialise it with the value.

Fails if the key already exists. If a lease is given, the key will automatically get deleted once it expires.

Parameters:

• path – Path to create

• value – Value to set

• lease – Lease to associate

Raises:

ConfigCollision if the key already exists

delete(path: str, must_exist: bool = True, recursive: bool = False, max_depth: int = 16, prefix: bool = False) → None

Delete the given key.

Parameters:

• path – Path of key to remove

• must_exist – Fail if path does not exist?

• recursive – Delete children keys at lower levels recursively (not used yet)

get(path: str) → str | None

Get value of a key.

Parameters:

path – Path of key to query

Returns:

Key value. None if it doesn’t exist.

list_keys(path: str, recurse: int | Iterable[int] = 0) → list[str]

List keys under given path.

Parameters:

• path – Prefix of keys to query. Append ‘/’ to list child paths.

• recurse – Children depths to include in search

Returns:

sorted key list

on_commit(callback: Callable[[], None]) → None

Register a callback to call when the transaction succeeds.

Exists mostly to enable test cases.

Parameters:

callback – Callback to call

reset(revision: DbRevision | None = None) → None

Reset the transaction, so it can be restarted after commit().

Parameters:

revision – to reset

Raises:

RuntimeError if the transaction is not committed.

property revision: int

The last-committed database revision.

Only valid to call after the transaction has been committed.

Returns:

revision from DbRevision

update(path: str, value: str) → None

Update an existing key.

Fails if the key does not exist.

Parameters:

• path – Path to update

• value – Value to set

Raises:

ConfigVanished if the key is not found

Etcd3 watcher

Etcd3 backend for SKA SDP configuration DB, implementating of caching watcher.

class ska_sdp_config.backend.etcd3_watcher.Etcd3TransactionWatcher(watcher: Etcd3Watcher, backend: Etcd3Backend, client: Etcd3Client, revision: DbRevision, max_retries: int = 64)

A series of queries and updates to be executed atomically.

This transaction offers the same interface as Etcd3Transaction, but utilises the watcher’s cache to return results more efficiently.

commit() → bool

Commit the transaction to the database.

This can fail, in which case the transaction must get reset and built again.

Returns:

Whether the commit succeeded

get(path: str) → str

Get value of a key.

Parameters:

path – Path of key to query

Returns:

Key value. None if it doesn’t exist.

list_keys(path: str, recurse: int = 0)

List keys under given path.

Parameters:

• path – Prefix of keys to query. Append ‘/’ to list child paths.

• recurse – Children depths to include in search

Returns:

sorted key list

reset(revision: DbRevision | None = None) → None

Reset the transaction, so it can be restarted after commit()

class ska_sdp_config.backend.etcd3_watcher.Etcd3Watcher(backend: Etcd3Backend, client: Etcd3Client, timeout: float | None = None, txn_wrapper: Callable[[Etcd3Transaction], object] | None = None, requery_progress: float = 0.2, max_retries: int = 15, retry_time: float = 0.1)

Watch for database changes by using nested transactions

Use as follows:

for watcher in config.watcher():
    for txn in watcher.txn():
        # ... do something
    for txn in watcher.txn():
        # ... do something else

At the end of a for loop iteration, the watcher will start watching all values read by transactions started through txn(), and only repeat the execution of the loop body once one of these values has changed.

trigger()

Manually triggers a loop

Can be called from a different thread to force a loop, even if the watcher is currently waiting.

txn(max_retries: int = 64) → Etcd3Transaction

Create nested transaction.

The watcher loop will iterate when any value read by transactions created by this method have changed in the database.

Note that these transactions otherwise behave exactly as normal transactions: As long as they are internally consistent, they will be commited. This means there is no consistency guarantees between transactions created from the same watcher, i.e. one transaction might read one value from the database while a later one reads another.

Parameters:

max_retries – Maximum number of times the transaction will be tried before giving up.

Memory backend

Memory backend for SKA SDP configuration DB.

The main purpose of this is for use in testing. In principle, it should behave in the same way as the etcd backend. No attempt has been made to make it thread-safe, so it probably isn’t.

class ska_sdp_config.backend.memory.MemoryBackend

In-memory backend implementation, principally for testing.

close() → None

Close the resource. This does nothing.

create(path: str, value: str, lease: Lease | None = None) → None

Create a key and initialise it with the value.

Parameters:

• path – Path to create

• value – Value to set

• lease – Lease to associate

Raises:

ConfigCollision if the key already exists

delete(path: str, must_exist: bool = True, recursive: bool = False, prefix: bool = False, max_depth: int = 16) → None

Delete the given key or key range.

Parameters:

• path – path (prefix) of keys to remove

• must_exist – Fail if path does not exist?

• recursive – Delete children keys at lower levels recursively

• max_depth – Recursion limit

• prefix – Delete all keys at given level with prefix

get(path: str, revision: DbRevision | None = None) → tuple[str, DbRevision]

Get value of a key.

Parameters:

• path – Path of key to query

• revision – to get

Returns:

value and revision

lease(ttl: float = 10) → Lease

Generate a dummy lease object.

Parameters:

ttl – time to live

Returns:

dummy lease object

list_keys(path: str, recurse: int = 0) → list[str]

Get a list of the keys at the given path.

In common with the etcd backend, the structure is “flat” rather than a real hierarchy, even though it looks like one.

Parameters:

• path – prefix of keys to query

• recurse – maximum recursion level to query

Returns:

list of keys

txn(max_retries: int = 64) → Iterable[MemoryTransaction]

Create an in-memory “transaction”.

Parameters:

max_retries – Maximum number of transaction loops

Returns:

transaction object

update(path: str, value: str) → None

Update an existing key. Fails if the key does not exist.

Parameters:

• path – Path to update

• value – New value of key

Raises:

ConfigVanished if the key does not exist

watcher(timeout: float | None = None, txn_wrapper: TxnWrapper | None = None) → Watcher

Create an in-memory “watcher”.

Parameters:

• timeout – timeout in seconds

• txn_wrapper – wrapper (factory) to return transaction

Returns:

MemoryWatcher object (mock of Etcd3Watcher)

class ska_sdp_config.backend.memory.MemoryTransaction(backend: Backend)

Transaction wrapper around the backend implementation.

Transactions always succeed if they are valid, so there is no need to loop; however the iterator is supported for compatibility with the etcd backend.

commit() → bool

Commit the transaction. This does nothing.

create(path: str, value: str, lease: Lease | None = None) → None

Create a key and initialise it with the value.

Fails if the key already exists. If a lease is given, the key will automatically get deleted once it expires.

Parameters:

• path – Path to create

• value – Value to set

• lease – Lease to associate

Raises:

ConfigCollision if the key already exists

delete(path: str, must_exist: bool = True, recursive: bool = False)

Delete the given key.

Parameters:

• path – Path of key to remove

• must_exist – Fail if path does not exist?

• recursive – Delete children keys at lower levels recursively (not used yet)

get(path: str) → str

Get value of a key.

Parameters:

path – Path of key to query

Returns:

Key value or None if it doesn’t exist.

list_keys(path: str, recurse: int = 0) → list[str]

List keys under given path.

Parameters:

• path – Prefix of keys to query. Append ‘/’ to list child paths.

• recurse – Children depths to include in search

Returns:

sorted key list

reset(revision: DbRevision | None = None) → None

Reset the transaction. This does nothing.

update(path: str, value: str) → None

Update an existing key.

Fails if the key does not exist.

Parameters:

• path – Path to update

• value – Value to set

Raises:

ConfigVanished if the key is not found

class ska_sdp_config.backend.memory.MemoryWatcher(backend: Backend, timeout: float | None = None, txn_wrapper: TxnWrapper | None = None)

Watcher wrapper around the backend implementation (Etcd3Watcher).

txn() → Iterable[MemoryTransaction]

Yield the wrapped MemoryTransaction object.

It does not implement the commit check that is part of Etcd3Watcher.txn(), hence it acts as MemoryBackend.txn()