1. Introduction

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crcPath is used to store the CRC of the on-disk storage file. As CRC is a linear function, it is possible to recompute CRC after changing a part of the file by xoring the old value of CRC with the old part of the file and the new part of the file (i.e. newCrc = oldCrc ⊕ crc(oldPart) ⊕ crc(newPart)). The file can be updated atomically by writing the new CRC value to a temporary file and replacing the old CRC file with a new one since renaming is an atomic operation. CRC file is used to detect corrupted data after a system crash.

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LatestMessagesFileStorage will be initialized in NodeRuntime.main using the provided configuration. As SafetyOracle depends on latest messages data structure, it will have to be initialized after LatestMessagesFileStorage.

3.5 Operational aspects

3.5.1 Metrics

LatestMessagesFileStorage will export the following metrics by reusing the existing class coop.rchain.metrics.Metrics:

• Accessing the in-memory representation map by invoking access method will be reported with name "latest-messages-file-storage-access".
• Appending a new key-value pair to the data file by invoking insert method will be reported with name "latest-messages-file-storage-insert".
• Overwriting the value of an existing key-value pair in the data file by invoking insert method will be reported with name "latest-messages-file-storage-update".

By using this metrics data one could determine how often lookups/updates/inserts are performed by node.

3.5.2 Logs

LatestMessagesFileStorage will not log anything.

3.6 Global software control

Both lookups and inserts/updates require acquiring the lock and are hence mutually exclusive. On the other hand, doing a bulk of lookups still requires taking only a single lock by using the access method.

3.

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7 Boundary conditions

• On node startup on-disk storage file is read into memory. In-memory representation and in-memory index are built from the read data.
• On node shutdown no additional actions are required as the system is always fully synchronized
• In case of node error, on startup the storage file is checked for corruptions by comparing the CRC to the actual check value of the storage file. In case of a corruption, the file is rebuilt from scratch using the information from persisted DAG.

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