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Update Spanner auto-gen layer. (#4033)
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| Spanner Client API | ||
| ================== | ||
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| .. automodule:: google.cloud.spanner_v1 | ||
| :members: | ||
| :inherited-members: |
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| .. | ||
| This page is pulled from the TransactionOption type, where this entire | ||
| kaboodle is auto-generated. Sphinx does not particularly appreciate | ||
| entire narrative documentation, complete with headers, in an arbitrary | ||
| class docstring, and complains about this, so I (lukesneeringer@) | ||
| manually copied it over here. | ||
| This should probably be updated when the Spanner code is re-generated. | ||
| This will be easy to remember because the source that needs to be copied | ||
| will be dropped in transaction_pb2.py and Sphinx will complain loudly | ||
| about it. | ||
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| Internal Google ticket: b/65243734 | ||
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| :orphan: | ||
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| .. _spanner-txn: | ||
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| Transactions | ||
| ============ | ||
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| Each session can have at most one active transaction at a time. After | ||
| the active transaction is completed, the session can immediately be | ||
| re-used for the next transaction. It is not necessary to create a new | ||
| session for each transaction. | ||
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| Transaction Modes | ||
| ================= | ||
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| Cloud Spanner supports two transaction modes: | ||
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| 1. Locking read-write. This type of transaction is the only way to write | ||
| data into Cloud Spanner. These transactions rely on pessimistic | ||
| locking and, if necessary, two-phase commit. Locking read-write | ||
| transactions may abort, requiring the application to retry. | ||
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| 2. Snapshot read-only. This transaction type provides guaranteed | ||
| consistency across several reads, but does not allow writes. Snapshot | ||
| read-only transactions can be configured to read at timestamps in the | ||
| past. Snapshot read-only transactions do not need to be committed. | ||
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| For transactions that only read, snapshot read-only transactions provide | ||
| simpler semantics and are almost always faster. In particular, read-only | ||
| transactions do not take locks, so they do not conflict with read-write | ||
| transactions. As a consequence of not taking locks, they also do not | ||
| abort, so retry loops are not needed. | ||
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| Transactions may only read/write data in a single database. They may, | ||
| however, read/write data in different tables within that database. | ||
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| Locking Read-Write Transactions | ||
| ------------------------------- | ||
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| Locking transactions may be used to atomically read-modify-write data | ||
| anywhere in a database. This type of transaction is externally | ||
| consistent. | ||
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| Clients should attempt to minimize the amount of time a transaction is | ||
| active. Faster transactions commit with higher probability and cause | ||
| less contention. Cloud Spanner attempts to keep read locks active as | ||
| long as the transaction continues to do reads, and the transaction has | ||
| not been terminated by [Commit][google.spanner.v1.Spanner.Commit] or | ||
| [Rollback][google.spanner.v1.Spanner.Rollback]. Long periods of | ||
| inactivity at the client may cause Cloud Spanner to release a | ||
| transaction's locks and abort it. | ||
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| Reads performed within a transaction acquire locks on the data being | ||
| read. Writes can only be done at commit time, after all reads have been | ||
| completed. Conceptually, a read-write transaction consists of zero or | ||
| more reads or SQL queries followed by | ||
| [Commit][google.spanner.v1.Spanner.Commit]. At any time before | ||
| [Commit][google.spanner.v1.Spanner.Commit], the client can send a | ||
| [Rollback][google.spanner.v1.Spanner.Rollback] request to abort the | ||
| transaction. | ||
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| Semantics | ||
| ~~~~~~~~~ | ||
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| Cloud Spanner can commit the transaction if all read locks it acquired | ||
| are still valid at commit time, and it is able to acquire write locks | ||
| for all writes. Cloud Spanner can abort the transaction for any reason. | ||
| If a commit attempt returns ``ABORTED``, Cloud Spanner guarantees that | ||
| the transaction has not modified any user data in Cloud Spanner. | ||
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| Unless the transaction commits, Cloud Spanner makes no guarantees about | ||
| how long the transaction's locks were held for. It is an error to use | ||
| Cloud Spanner locks for any sort of mutual exclusion other than between | ||
| Cloud Spanner transactions themselves. | ||
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| Retrying Aborted Transactions | ||
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | ||
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| When a transaction aborts, the application can choose to retry the whole | ||
| transaction again. To maximize the chances of successfully committing | ||
| the retry, the client should execute the retry in the same session as | ||
| the original attempt. The original session's lock priority increases | ||
| with each consecutive abort, meaning that each attempt has a slightly | ||
| better chance of success than the previous. | ||
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| Under some circumstances (e.g., many transactions attempting to modify | ||
| the same row(s)), a transaction can abort many times in a short period | ||
| before successfully committing. Thus, it is not a good idea to cap the | ||
| number of retries a transaction can attempt; instead, it is better to | ||
| limit the total amount of wall time spent retrying. | ||
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| Idle Transactions | ||
| ~~~~~~~~~~~~~~~~~ | ||
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| A transaction is considered idle if it has no outstanding reads or SQL | ||
| queries and has not started a read or SQL query within the last 10 | ||
| seconds. Idle transactions can be aborted by Cloud Spanner so that they | ||
| don't hold on to locks indefinitely. In that case, the commit will fail | ||
| with error ``ABORTED``. | ||
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| If this behavior is undesirable, periodically executing a simple SQL | ||
| query in the transaction (e.g., ``SELECT 1``) prevents the transaction | ||
| from becoming idle. | ||
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| Snapshot Read-Only Transactions | ||
| ------------------------------- | ||
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| Snapshot read-only transactions provides a simpler method than locking | ||
| read-write transactions for doing several consistent reads. However, | ||
| this type of transaction does not support writes. | ||
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| Snapshot transactions do not take locks. Instead, they work by choosing | ||
| a Cloud Spanner timestamp, then executing all reads at that timestamp. | ||
| Since they do not acquire locks, they do not block concurrent read-write | ||
| transactions. | ||
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| Unlike locking read-write transactions, snapshot read-only transactions | ||
| never abort. They can fail if the chosen read timestamp is garbage | ||
| collected; however, the default garbage collection policy is generous | ||
| enough that most applications do not need to worry about this in | ||
| practice. | ||
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| Snapshot read-only transactions do not need to call | ||
| [Commit][google.spanner.v1.Spanner.Commit] or | ||
| [Rollback][google.spanner.v1.Spanner.Rollback] (and in fact are not | ||
| permitted to do so). | ||
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| To execute a snapshot transaction, the client specifies a timestamp | ||
| bound, which tells Cloud Spanner how to choose a read timestamp. | ||
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| The types of timestamp bound are: | ||
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| - Strong (the default). | ||
| - Bounded staleness. | ||
| - Exact staleness. | ||
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| If the Cloud Spanner database to be read is geographically distributed, | ||
| stale read-only transactions can execute more quickly than strong or | ||
| read-write transaction, because they are able to execute far from the | ||
| leader replica. | ||
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| Each type of timestamp bound is discussed in detail below. | ||
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| Strong | ||
| ~~~~~~ | ||
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| Strong reads are guaranteed to see the effects of all transactions that | ||
| have committed before the start of the read. Furthermore, all rows | ||
| yielded by a single read are consistent with each other -- if any part | ||
| of the read observes a transaction, all parts of the read see the | ||
| transaction. | ||
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| Strong reads are not repeatable: two consecutive strong read-only | ||
| transactions might return inconsistent results if there are concurrent | ||
| writes. If consistency across reads is required, the reads should be | ||
| executed within a transaction or at an exact read timestamp. | ||
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| See | ||
| [TransactionOptions.ReadOnly.strong][google.spanner.v1.TransactionOptions.ReadOnly.strong]. | ||
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| Exact Staleness | ||
| ~~~~~~~~~~~~~~~ | ||
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| These timestamp bounds execute reads at a user-specified timestamp. | ||
| Reads at a timestamp are guaranteed to see a consistent prefix of the | ||
| global transaction history: they observe modifications done by all | ||
| transactions with a commit timestamp <= the read timestamp, and observe | ||
| none of the modifications done by transactions with a larger commit | ||
| timestamp. They will block until all conflicting transactions that may | ||
| be assigned commit timestamps <= the read timestamp have finished. | ||
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| The timestamp can either be expressed as an absolute Cloud Spanner | ||
| commit timestamp or a staleness relative to the current time. | ||
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| These modes do not require a "negotiation phase" to pick a timestamp. As | ||
| a result, they execute slightly faster than the equivalent boundedly | ||
| stale concurrency modes. On the other hand, boundedly stale reads | ||
| usually return fresher results. | ||
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| See | ||
| [TransactionOptions.ReadOnly.read\_timestamp][google.spanner.v1.TransactionOptions.ReadOnly.read\_timestamp] | ||
| and | ||
| [TransactionOptions.ReadOnly.exact\_staleness][google.spanner.v1.TransactionOptions.ReadOnly.exact\_staleness]. | ||
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| Bounded Staleness | ||
| ~~~~~~~~~~~~~~~~~ | ||
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| Bounded staleness modes allow Cloud Spanner to pick the read timestamp, | ||
| subject to a user-provided staleness bound. Cloud Spanner chooses the | ||
| newest timestamp within the staleness bound that allows execution of the | ||
| reads at the closest available replica without blocking. | ||
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| All rows yielded are consistent with each other -- if any part of the | ||
| read observes a transaction, all parts of the read see the transaction. | ||
| Boundedly stale reads are not repeatable: two stale reads, even if they | ||
| use the same staleness bound, can execute at different timestamps and | ||
| thus return inconsistent results. | ||
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| Boundedly stale reads execute in two phases: the first phase negotiates | ||
| a timestamp among all replicas needed to serve the read. In the second | ||
| phase, reads are executed at the negotiated timestamp. | ||
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| As a result of the two phase execution, bounded staleness reads are | ||
| usually a little slower than comparable exact staleness reads. However, | ||
| they are typically able to return fresher results, and are more likely | ||
| to execute at the closest replica. | ||
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| Because the timestamp negotiation requires up-front knowledge of which | ||
| rows will be read, it can only be used with single-use read-only | ||
| transactions. | ||
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| See | ||
| [TransactionOptions.ReadOnly.max\_staleness][google.spanner.v1.TransactionOptions.ReadOnly.max\_staleness] | ||
| and | ||
| [TransactionOptions.ReadOnly.min\_read\_timestamp][google.spanner.v1.TransactionOptions.ReadOnly.min\_read\_timestamp]. | ||
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| Old Read Timestamps and Garbage Collection | ||
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | ||
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| Cloud Spanner continuously garbage collects deleted and overwritten data | ||
| in the background to reclaim storage space. This process is known as | ||
| "version GC". By default, version GC reclaims versions after they are | ||
| one hour old. Because of this, Cloud Spanner cannot perform reads at | ||
| read timestamps more than one hour in the past. This restriction also | ||
| applies to in-progress reads and/or SQL queries whose timestamp become | ||
| too old while executing. Reads and SQL queries with too-old read | ||
| timestamps fail with the error ``FAILED_PRECONDITION``. |
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| Spanner Client Types | ||
| =================================== | ||
|
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||
| .. automodule:: google.cloud.spanner_v1.types | ||
| :members: |
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