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// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
package com.yahoo.vespa.hosted.node.admin.component;
/**
* This class is thread unsafe: All method calls MUST be exclusive and serialized.
*
* In a specialized environment it is possible to provide a richer context than TaskContext:
* - Define a subclass T of TaskContext with the additional functionality.
* - Define task classes that implement IdempotentTask<T>.
*/
public interface IdempotentTask<T extends TaskContext> {
/**
* A short id of the task to e.g. identify the task in the log.
*
* Prefer PascalCase and without white-space.
*
* Example: "EnableDocker"
*/
default String name() { return getClass().getSimpleName(); }
/**
* Execute an administrative task to converge towards some ideal state, whether it is
* system state or in-memory Java state.
*
* converge() must be idempotent: it may be called any number of times, or
* interrupted at any time e.g. by `kill -9`.
*
* converge() is not thread safe: The caller must ensure there is at most one invocation
* of converge() at any given time.
*
* @return false if already converged, i.e. was a no-op. A typical sequence of converge()
* calls on a IdempotentTask will consist of:
* - Any number of calls that throws an exception due to some issues. Assuming
* no exceptions were thrown, or the issue eventually resolved itself...
* (convergence failure)
* - Returns true once (converged just now)
* - Returns false for all further calls (already converged)
* @throws RuntimeException (or a subclass) if the task is unable to converge.
*/
boolean converge(T context);
}
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