Transactions

Transactions are not encrypted, so it is possible to browse and view every transaction ever collected into a block. Once transactions are buried under enough confirmations they can be considered irreversible. Standard transaction outputs nominate addresses , and the redemption of any future inputs requires a relevant signature. All transactions are visible in the block chain , and can be viewed with a hex editor. A block chain browser is a site where every transaction included within the block chain can be viewed in human-readable terms. This is useful for seeing the technical details of transactions in action and for verifying payments.

The input in this transaction imports 50 BTC from output 0 in transaction f5d Then the output sends 50 BTC to a Bitcoin address expressed here in hexadecimal When the recipient wants to spend this money, he will reference output 0 of this transaction in an input of his own transaction. An input is a reference to an output from a previous transaction. Multiple inputs are often listed in a transaction. All of the new transaction's input values that is, the total coin value of the previous outputs referenced by the new transaction's inputs are added up, and the total less any transaction fee is completely used by the outputs of the new transaction.

Previous tx is a hash of a previous transaction. Index is the specific output in the referenced transaction. ScriptSig is the first half of a script discussed in more detail later. The script contains two components, a signature and a public key. The public key must match the hash given in the script of the redeemed output.

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The public key is used to verify the redeemers signature, which is the second component. More precisely, the second component is an ECDSA signature over a hash of a simplified version of the transaction. It, combined with the public key, proves the transaction was created by the real owner of the address in question. Various flags define how the transaction is simplified and can be used to create different types of payment.

An output contains instructions for sending bitcoins. ScriptPubKey is the second half of a script discussed later. There can be more than one output, and they share the combined value of the inputs. Because each output from one transaction can only ever be referenced once by an input of a subsequent transaction, the entire combined input value needs to be sent in an output if you don't want to lose it.

Any input bitcoins not redeemed in an output is considered a transaction fee ; whoever generates the block can claim it by inserting it into the coinbase transaction of that block. To verify that inputs are authorized to collect the values of referenced outputs, Bitcoin uses a custom Forth-like scripting system.

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The input's scriptSig and the referenced output's scriptPubKey are evaluated in that order , with scriptPubKey using the values left on the stack by scriptSig. The input is authorized if scriptPubKey returns true.

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Through the scripting system, the sender can create very complex conditions that people have to meet in order to claim the output's value. For example, it's possible to create an output that can be claimed by anyone without any authorization. It's also possible to require that an input be signed by ten different keys, or be redeemable with a password instead of a key. However starting with Redis 2. The new behavior makes it much more simple to mix transactions with pipelining, so that the whole transaction can be sent at once, reading all the replies later at once.

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Errors happening after EXEC instead are not handled in a special way: This is more clear on the protocol level. In the following example one command will fail when executed even if the syntax is right:. It's up to the client library to find a sensible way to provide the error to the user.

It's important to note that even when a command fails, all the other commands in the queue are processed — Redis will not stop the processing of commands.

transaction

Another example, again using the wire protocol with telnet , shows how syntax errors are reported ASAP instead:. This time due to the syntax error the bad INCR command is not queued at all.

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• Transaction.
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If you have a relational databases background, the fact that Redis commands can fail during a transaction, but still Redis will execute the rest of the transaction instead of rolling back, may look odd to you. An argument against Redis point of view is that bugs happen, however it should be noted that in general the roll back does not save you from programming errors.

For instance if a query increments a key by 2 instead of 1, or increments the wrong key, there is no way for a rollback mechanism to help. Given that no one can save the programmer from his or her errors, and that the kind of errors required for a Redis command to fail are unlikely to enter in production, we selected the simpler and faster approach of not supporting roll backs on errors.

In this case, no commands are executed and the state of the connection is restored to normal. WATCH ed keys are monitored in order to detect changes against them. For example, imagine we have the need to atomically increment the value of a key by 1 let's suppose Redis doesn't have INCR. This will work reliably only if we have a single client performing the operation in a given time. If multiple clients try to increment the key at about the same time there will be a race condition.

For instance, client A and B will read the old value, for instance, The value will be incremented to 11 by both the clients, and finally SET as the value of the key. So the final value will be 11 instead of Using the above code, if there are race conditions and another client modifies the result of val in the time between our call to WATCH and our call to EXEC , the transaction will fail.

We just have to repeat the operation hoping this time we'll not get a new race. This form of locking is called optimistic locking and is a very powerful form of locking. In many use cases, multiple clients will be accessing different keys, so collisions are unlikely — usually there's no need to repeat the operation. It is a command that will make the EXEC conditional: But they might be changed by the same client inside the transaction without aborting it. Otherwise the transaction is not entered at all. WATCH can be called multiple times. Sometimes this is useful as we optimistically lock a few keys, since possibly we need to perform a transaction to alter those keys, but after reading the current content of the keys we don't want to proceed.

A good example to illustrate how WATCH can be used to create new atomic operations otherwise not supported by Redis is to implement ZPOP, that is a command that pops the element with the lower score from a sorted set in an atomic way. This is the simplest implementation:.

Transactions – Redis

If EXEC fails i. A Redis script is transactional by definition, so everything you can do with a Redis transaction, you can also do with a script, and usually the script will be both simpler and faster.

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This duplication is due to the fact that scripting was introduced in Redis 2.