工作立异情势

Undo the work performed by a series of steps, which together define an
eventually consistent operation, if one or more of the steps fail.
Operations that follow the eventual consistency model are commonly found
in cloud-hosted applications that implement complex business processes
and workflows.

撤回由一多元步骤执行的干活,一起定义一个说到底一致的操作,假如一个或七个步骤败北。在云托管的应用程序,完成复杂的工作和行事流程,平日操作是按照的末梢一致性模型。

Context and Problem 背景和题材

Applications running in the cloud frequently modify data. This data may
be spread across an assortment of data sources held in a variety of
geographic locations. To avoid contention and improve performance in a
distributed environment such as this, an application should not attempt
to provide strong transactional consistency. Rather, the application
should implement eventual consistency. In this model, a typical
business operation consists of a series of autonomous steps. While these
steps are being performed the overall view of the system state may be
inconsistent, but when the operation has completed and all of the steps
have been executed the system should become consistent again.

在云端运行的应用程序平常修改数据,此数量或者会散开在分歧地理地点的多少源中。为了防止竞争和增加质量,如在分布式环境中,应用程序不应当统计提供强劲的政工一致性。相反,应用程序应该达成最后的一致性。在那一个模型中,一个一级的业务操作由一各类的独立步骤组成。即便那一个手续正在拓展系统状态的总体视图可能是分化的,但当操作已经到位,并已执行的具有手续的系统应成为平等。

Note:

The 必发bifa88手机客服端,Data Consistency
Primer
provides
more information about why distributed transactions do not scale well,
and the principles that underpin the eventual consistency model.

A significant challenge in the eventual consistency model is how to
handle a step that has failed irrecoverably. In this case it may be
necessary to undo all of the work completed by the previous steps in the
operation. However, the data cannot simply be rolled back because other
concurrent instances of the application may have since changed it. Even
in cases where the data has not been changed by a concurrent instance,
undoing a step might not simply be a matter of restoring the original
state. It may be necessary to apply various business-specific rules (see
the travel website described in the
Example
section).

在终极一致性模型的一个紧要的挑衅是怎样处理一个破产的步调。在那种场所下,它恐怕是少不了的吊销所有的行事成就的前一步操作。但是,数据不可能大概地回滚,因为应用程序的此外并发实例可能已经变更了它。就算没有得到数码的出现情形的更动,裁撤一步可能不只是过来原先的景况。它可能是少不了的,适用于各类工作的有血有肉规则(见例子)。

If an operation that implements eventual consistency spans several
heterogeneous data stores, undoing the steps in such an operation will
require visiting each data store in turn. The work performed in every
data store must be undone reliably to prevent the system from remaining
inconsistent.

假诺落成最后一致性跨越来越多少个异构数据存储操作,打消步骤等操作须求种种走访每个数据存储。在种种数据存储中执行的干活都不可以不被收回,以幸免系统的不平等。

Not all data affected by an operation that implements eventual
consistency might be held in a database. In a Service Oriented
Architecture (SOA) environment an operation may invoke an action in a
service, and cause a change in the state held by that service. To undo
the operation, this state change must also be undone. This may involve
invoking the service again and performing another action that reverses
the effects of the first.

并不是享有的数额都会影响到结尾的一致性,可能在一个数据库中开展。在面向服务的连串布局(SOA)环境下的操作可能在劳务调用动作,而造成的劳动情况的变迁。撤销该操作,该意况更改也不可以不被裁撤。那说不定涉嫌再度调用服务,并举行另一个五花大绑的影响的走动。

Solution 解决方案

Implement a compensating transaction. The steps in a compensating
transaction must undo the effects of the steps in the original
operation. A compensating transaction might not be able to simply
replace the current state with the state the system was in at the start
of the operation because this approach could overwrite changes made by
other concurrent instances of an application. Rather, it must be an
intelligent process that takes into account any work done by concurrent
instances. This process will usually be application-specific, driven by
the nature of the work performed by the original operation.

落到实处补充工作。补偿工作中的步骤必须撤废原操作步骤的影响。一个互补工作可能无法简单地更迭当前运作状态的图景,因为那种办法可以覆盖应用程序的其余并发实例举行更改。相反,它必须是一个智能的长河,需求考虑到其余工作所做的出现实例。那一个进程一般是特定于应用程序的,由原操作实施的干活的特性驱动的。

A common approach to implementing an eventually consistent operation
that requires compensation is to use a workflow. As the original
operation proceeds, the system records information about each step and
how the work performed by that step can be undone. If the operation
fails at any point, the workflow rewinds back through the steps it has
completed and performs the work that reverses each step. Note that a
compensating transaction might not have to undo the work in the exact
mirror-opposite order of the original operation, and it may be possible
to perform some of the undo steps in parallel.

一种普遍的不二法门来促成一个最终一致的操作,必要补给操作是使用一个工作流。作为原有的操作进行,系统记录有关每一步的音信,以及怎样形成该步骤所执行的干活。如若在任曾几何时候操作失利,工作流将重返到步骤已经达成和施行工作的每一步。请小心,补偿工作可能不要求在原本的操作的镜像相反的相继中收回工作,并可能有可能举行一些互相的打消步骤。

Note:

This approach is similar to the Sagas strategy. A description of this
strategy is available online in Clemens Vasters’
blog
.

A compensating transaction is itself an eventually consistent operation
and it could also fail. The system should be able to resume the
compensating transaction at the point of failure and continue. It may be
necessary to repeat a step that has failed, so the steps in a
compensating transaction should be defined as idempotent commands. For
more information about idempotency, see Idempotency
Patterns

on Jonathan Oliver’s blog.

补偿性事务本身就是一种最后一致的操作,也说不定破产。该连串应该力所能及恢复生机在故障点的补偿性操作,并继续。可能须要重新失利步骤,那么在补偿工作的步子应定义为幂等的授命。关于幂等性的越来越多信息,参见Idempotency
Patterns

on Jonathan Oliver’s blog。

In some cases it may not be possible to recover from a step that has
failed except through manual intervention. In these situations the
system should raise an alert and provide as much information as possible
about the reason for the failure.

在某些情状下,它可能依旧不能恢复生机从一个退步的步骤,除了通过人为干预。在这种场合下,系统应该提供预警,并尽量为战败的缘由提供多的新闻。

Issues and Considerations 难题和注意事项

Consider the following points when deciding how to implement this
pattern:

在支配哪些贯彻那些方式时,考虑以下几点:

  • It might not be easy to determine when a step in an operation that
    implements eventual consistency has failed. A step might not fail
    immediately, but instead it could block. It may be necessary to
    implement some form of time-out mechanism.
  • 那或许不不难决断,在一个末尾一致性操作的某步骤的操作失败。一个手续可能不会立即战败,但它反而被堵塞。它可能需求贯彻某种方式的超时机制。
  • Compensation logic is not easily generalized. A compensating
    transaction is application-specific; it relies on the application
    having sufficient information to be able to undo the effects of each
    step in a failed operation.
  • 补偿论理是不易于普及的。补偿工作是应用程序特定的,它借助于应用程序有丰裕的信息,可以裁撤一回败北的操作中的每一步的影响。
  • You should define the steps in a compensating transaction as
    idempotent commands. This enables the steps to be repeated if the
    compensating transaction itself fails.
  • 您应该定义一个补充工作为幂等的命令。那使得可以重新的手续,假使补偿本身败北。
  • The infrastructure that handles the steps in the original operation,
    and the compensating transaction, must be resilient. It must not
    lose the information required to compensate for a failing step, and
    it must be able to reliably monitor the progress of the compensation
    logic.
  • 处理在原来操作中的步骤和增补工作的基础必须是有弹性的。它不可能失去所需的音信以弥补一个功败垂成的步子,它必须可以可信地监测补偿论理的进度。
  • A compensating transaction does not necessarily return the data in
    the system to the state it was in at the start of the original
    operation. Instead, it compensates for the work performed by the
    steps that completed successfully before the operation failed.
  • 补给工作不必再次来到到原有操作起来时的种类中的数据。相反,它弥补了工作达成的步调,在操作退步从前到位的做事。
  • The order of the steps in the compensating transaction does not
    necessarily have to be the mirror opposite of the steps in the
    original operation. For example, one data store may be more
    sensitive to inconsistencies than another, and so the steps in the
    compensating transaction that undo the changes to this store should
    occur first.
  • 在补偿工作中步骤的顺序不必然是在原本的操作步骤的镜像。例如,一个数目存储可能比另一个进一步灵活,由此,在补充工作中,撤销对该存储的转移的步调应该首头阵出。
  • Placing a short-term timeout-based lock on each resource that is
    required to complete an operation, and obtaining these resources in
    advance, can help increase the likelihood that the overall activity
    will succeed. The work should be performed only after all the
    resources have been acquired. All actions must be finalized before
    the locks expire.
  • 在每一个资源上停放一个短时间的超时锁,来已毕一个操作,并提早得到这一个资源,可以扶助扩张全体活动的可能。所有的资源都被买断后,才开展那工作。所有行动必须在锁到期前完毕。
  • Consider using retry logic that is more forgiving than usual to
    minimize failures that trigger a compensating transaction. If a step
    in an operation that implements eventual consistency fails, try
    handling the failure as a transient exception and repeat the step.
    Only abort the operation and initiate a compensating transaction if
    a step fails repeatedly or irrecoverably.
  • 考虑选取重试逻辑,常常是的更易于的,以收缩故障,触发一个填补工作。如若在一个操作中落到实处最后的一致性的步子失利,试着处理故障作为一个暂时的丰硕,然后再一次步骤。只有废弃操作,即便一个手续失利或者反复地开行补偿工作。

Note:

Many of the challenges and issues of implementing a compensating
transaction are the same as those concerned with implementing eventual
consistency. See the section Considerations for Implementing Eventual
Consistency in the Data Consistency
Primer
for more
information.

When to Use this Pattern 曾几何时利用那种形式

Use this pattern only for operations that must be undone if they fail.
If possible, design solutions to avoid the complexity of requiring
compensating transactions (for more information, see the Data
Consistency
Primer
).

使用此情势仅用于操作必须是要是她们无法收回。如若可能的话,设计缓解方案以幸免复杂的渴求补偿工作(更多音讯,见数据一致性引物)。

Example 例子

A travel website enables customers to book itineraries. A single
itinerary may comprise a series of flights and hotels. A customer
traveling from Seattle to London and then on to Paris could perform the
following steps when creating an itinerary:

一个出境游网站,使客户预订行程。一个十足的路程可概括一多重的航班和客栈。一位顾客从明尼阿波莉斯到London,然后到巴黎,在创设行程时,可以实施以下步骤:

  1. Book a seat on flight F1 from Seattle to London.
  2. Book a seat on flight F2 from London to Paris.
  3. Book a seat on flight F3 from Paris to Seattle.
  4. Reserve a room at hotel H1 in London.
  5. Reserve a room at hotel H2 in Paris.

预约一张从青岛到London的航班F1。
预定一张从London到法国巴黎的航班F2。
预定一张从法国首都到路易港的航班F3。
储备在London一间旅馆H1。
在法国首都餐馆预定一个房间。

These steps constitute an eventually consistent operation, although each
step is essentially a separate atomic action in its own right.
Therefore, as well as performing these steps, the system must also
record the counter operations necessary to undo each step in case the
customer decides to cancel the itinerary. The steps necessary to perform
the counter operations can then run as a compensating transaction if
necessary.

这几个步骤构成末了一致性操作,尽管每一步基本上是在团结的义务单独的原子动作。因而,以及在进行那么些步骤时,系统还非得记录要求撤销在场合下,客户决定收回行程的种种步骤中的计数器的操作。然后必须履行计数器操作步骤可以在须求时运行补偿性事务。

Notice that the steps in the compensating transaction might not be the
exact opposite of the original steps, and the logic in each step in the
compensating transaction must take into account any business-specific
rules. For example, “unbooking” a seat on a flight might not entitle the
customer to a complete refund of any money paid.

请留意,在补偿工作的步骤可能不是本来的步骤完全相反,并且在补偿工作的种种步骤都必须考虑到其余特定的事务规则逻辑。例如,“订舱”在航班上的座位可能没有资格客户开发的别样款项全额退款。

必发bifa88手机客服端 1

Figure 1 – Generating a compensating transaction to undo a long-running
transaction to book a travel itinerary

图1 – 生成补偿事务撤销长日子运作的业务预约旅游行程

Note:

It may be possible for the steps in the compensating transaction to be
performed in parallel, depending on how you have designed the
compensating logic for each step.

有可能在补充工作的步骤并行取决于你什么样设计用来每个步骤中的补偿论理被执行。

In many business solutions, failure of a single step does not always
necessitate rolling the system back by using a compensating transaction.
For example, if—after having booked flights F1, F2, and F3 in the travel
website scenario—the customer is unable to reserve a room at hotel H1,
it is preferable to offer the customer a room at a different hotel in
the same city rather than cancelling the flights. The customer may still
elect to cancel (in which case the compensating transaction runs and
undoes the bookings made on flights F1, F2, and F3), but this decision
should be made by the customer rather than by the system.

在很多工作解决方案,单步的破产并不总是必不可少选用补偿工作滚动系统苏醒。例如,具有预订航班F1,F2和F3在骑行网站情景客户不可以预定时酒馆H1假使-之后,最好是能为客户提供在同一个都会的屋子在不相同的酒店而不是吊销航班。客户照旧可以选拔废除(在那种状态下,补偿工作中运行,并注销关于航班F1,F2和F3作出的订购),但该决定应由用户,而不是由系统开展。

Related Patterns and Guidance 相关情势和引导

The following patterns and guidance may also be relevant when
implementing this pattern:

  • Data Consistency
    Primer
    . The
    Compensating Transaction pattern is frequently used to undo
    operations that implement the eventual consistency model. This
    primer provides more information on the benefits and tradeoffs of
    eventual consistency.
  • Scheduler-Agent-Supervisor
    Pattern
    .
    This pattern describes how to implement resilient systems that
    perform business operations that utilize distributed services and
    resources. In some circumstances, it may be necessary to undo the
    work performed by an operation by using a compensating transaction.
  • Retry
    Pattern
    .
    Compensating transactions can be expensive to perform, and it may be
    possible to minimize their use by implementing an effective policy
    of retrying failing operations by following the Retry pattern.

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