diff --git a/spec/1_design_goals.md b/spec/1_design_goals.md index c1fca5be5..a34438707 100644 --- a/spec/1_design_goals.md +++ b/spec/1_design_goals.md @@ -1,12 +1,12 @@ -# Chronicle Map design goals, premises and guarantees +# Chronicle Map Design Goals, Assumptions and Guarantees - Chronicle Map is an in-memory, highly concurrent key-value store. Generally keys and values are arbitrary sequences of bytes. - - Chronicle Map optionally persists to a *single file* via memory-mapping facility, present in + - Chronicle Map optionally persists to a *single file* via the memory-mapping facility, present in Windows and POSIX-compatible operation systems (`mmap`). The file shouldn't necessarily materialize - on disk, it's OK for Chronicle Map if it resides a memory-mounted FS. The whole Chronicle Map state - is contained in the file contents, i. e. it is possible to move, copy or send the file to another - machine, access it observe exactly the same Chronicle Map state. Chronicle Map doesn't use the file + on disk. It's OK for Chronicle Map if the file resides a memory-mounted FS. The whole Chronicle Map state + is contained in the file contents i. e. it is possible to move, copy or send the file to another + machine, access it and observe exactly the same Chronicle Map state. Chronicle Map doesn't use the file metadata. - Chronicle Map supports fully-featured concurrent access from multiple processes, mapping the same file. @@ -21,18 +21,18 @@ #### CPU - CPU should support atomic 32-bit compare-and-swap operations with aligned memory, i. e. if - several threads, possibly belong to different processes, try to perform compare-and-swap + several threads, possibly belonging to different processes, try to perform compare-and-swap operation on the same 4-byte block, one thread succeeds and all the rest fail. - Aligned 32-bit or 64-bit writes should be atomic, that means either all 4(8) bytes are written to memory, or nothing is written. - If some values Xi are written at addresses Ai, then 32- or 64-bit value Y - is written at address B, there should be a way to ensure some memory order, in particular, if a + is written at address B, there should be a way to ensure some memory order. In particular, if a concurrent thread reads value Y from address B, then it should read Xii addresses (but not the old values, stored at these addresses). #### Memory-mapping -Chronicle Map assumes that the memory-mapping implementation doesn't corrupt the mapped file, even +Chronicle Map assumes that the memory-mapping implementation does not corrupt the mapped file, even if operation system execution was interrupted in any way (black out, virtual machine crash, etc.) It means Chronicle Map expects some memory might be "stale", i. e. the bits written shortly before operation system failure might not be persisted, but it doesn't expect to read the bit values that @@ -40,12 +40,12 @@ have never been written to the memory. On the other hand, it doesn't assume anything specific about how memory if flushed to disk: - - The order, in which memory is flushed (up or down by addresses, random, concurrent, etc.) - - The minimum persistence atomicity, i. e. Chronicle Map doesn't assume memory blocks of any size + - The order in which memory is flushed (up or down by addresses, random, concurrent, etc.) + - The minimum persistence atomicity i.e. Chronicle Map doesn't assume memory blocks of any size should be either fully written to disk or not written at all, down to a single bit. - - The maximum timeout dirtied memory could be not flushed to disk, assumes some memory could remain + - The maximum timeout dirty memory could not be flushed to disk. i.e. it assumes some memory could remain dirty forever. - - The maximum fraction of dirty memory not written to disk, assumes that all mapped memory could be + - The maximum fraction of dirty memory not written to disk i.e. it assumes that all mapped memory could be dirty. ### Guarantees @@ -53,11 +53,11 @@ On the other hand, it doesn't assume anything specific about how memory if flush If the above assumptions are met, Chronicle Map aims to satisfy the following guarantees: - Under any conditions (concurrent access, inter-process access, access after operation system - failures) and using any type of access: querying entry by the key or iteration, only entries that + failures) and using any type of access (querying the entry by the key or iteration), only entries that were ever stored in the Chronicle Map could be observed. Reading corrupted or half written values - by some keys, or observing some never stored keys during iteration is disallowed. - - If Chronicle Map is persisted to a file and operation system fails to flush all dirty memory to - disk due to power-off or any other failure, when the file is mapped to memory and accessed again, + by some keys, or observing some keys which were never stored during the iteration is disallowed. + - If Chronicle Map is persisted to a file and the operation system fails to flush all dirty memory to + disk due to a power-off or any other failure, when the file is mapped to memory and accessed again, some values associated with the keys shortly before the failure could be lost. - If some entry is successfully put into the Chronicle Map, and, if the Chronicle Map is persisted to disk, all memory that is dirty by the end of this operation is successfully flushed, this entry @@ -67,42 +67,42 @@ If the above assumptions are met, Chronicle Map aims to satisfy the following gu The ultimate goal of Chronicle Map design is efficiency: - - If number of entries in Chronicle Map is much greater than the number of accessor hardware + - If the number of entries in Chronicle Map is much greater than the number of accessor hardware threads, it should scale well up to the total number of hardware threads present in the system. - - Chronicle Map should be a low-latency key-value store, in the sense that for any particular - percentile (90%, 99%, 99.9% etc.) latency of particular operations with Chronicle Map should be the - best or among the best compared to latencies of other key-value stores filled up with the same data + - Chronicle Map should be a low-latency key-value store, meaning that for any particular + percentile (90%, 99%, 99.9% etc.) the latency of Chronicle Map operations should be the + best or among the best compared to latencies of other key-value stores filled with the same data and providing similar guarantees. - Chronicle Map shouldn't severely overuse memory and disk space for any range and distribution of - key and values sizes. Chronicle Map shows best-in-class memory footprint if keys and values are + key and values sizes. Chronicle Map exhibits best-in-class memory footprint if keys and values are small and fixed-sized. -### Premises +### Assumptions -Chronicle Map targets the goals outlined above under the following premises: +Chronicle Map targets the goals outlined above under the following assumptions: - It runs on AMD64 (aka x86_64) architecture - It runs on Linux - The total volume of memory, used by Chronicle Map fits the available physical memory. In other words, the memory accessed by Chronicle Map should never be swapped to disk. -If these conditions are not held, Chronicle Map must remain operational, but it shouldn't be +If these assumptions are not held, Chronicle Map must remain operational, but it shouldn't be expected to be very efficient. ## Non-goals - Alphabetical key ordering (sorted keys). - Durability. There are ways to make Chronicle Map look like a durable key-value store, e. g. - performing `msync` after each operation, but you better take a database designed for durability. + performing `msync` after each operation. However, if required, it would be better use a database designed for durability. -The following items are non-goals yet, but might become goals in the future, at least under certain -conditions or with additional premises: +The following items are non-goals but might become goals in the future, at least under certain +conditions or with additional assumptions: - Entry ordering by insertion, update or access. - - Hard latency guarantees. Chronicle Map doesn't say "all operations complete in less than one + - Hard latency guarantees. e.g. Chronicle Map doesn't specify "all operations complete in less than one second." Persistence via memory-mapping implies that any memory write could trigger flushing the whole file to disk, that could take long time. - - Entry expirations. It could be emulated by storing last update (access) time in the value bytes - and checking it on each access to Chronicle Map, and possibly running a background thread that - walks though the Chronicle Map and cleans expired entries, but it is not guaranteed that Chronicle - Map would perform better than a cache with "native" expirations. \ No newline at end of file + - Entry expirations. This could be emulated by storing the last update (access) time in the value bytes + and checking it on each access to Chronicle Map. This could be implemented, for example, by running a background thread that + traverses the Chronicle Map and cleans expired entries. However it is not guaranteed that Chronicle + Map would perform better than a cache with "native" expirations.