Project milestone 3: SurfStore
In this project you are going to create a cloud-based file storage service called SurfStore.
SurfStore is a networked file storage application that is based on Dropbox, and lets you sync
files to and from the “cloud”. You will implement the cloud service, and a client which interacts
with your service via RPC.
Multiple clients can concurrently connect to the SurfStore service to access a common, shared
set of files. Clients accessing SurfStore “see” a consistent set of updates to files, but SurfStore
does not offer any guarantees about operations across files, meaning that it does not support
multi-file transactions (such as atomic move).
The SurfStore service is composed of the following two services:
● BlockStore: The content of each file in SurfStore is divided up into chunks, or blocks,
each of which has a unique identifier. This service stores these blocks, and when given
an identifier, retrieves and returns the appropriate block.
● MetadataStore: The MetadataStore service holds the mapping of filenames to blocks.
We now describe the service in more detail.
Blocks, hashes, and hashlists
A file in SurfStore is broken into an ordered sequence of one or more blocks. Each block is of
uniform size (defined by the command line argument), except for the last block in the file, which
may be smaller (but must be at least 1 byte large). As an example, assume the block size is
4096 bytes, and consider the following file:
The file ‘MyFile.mp4’ is 14,437 bytes long, and the block size is 4KB. The file is broken into
blocks b0, b1, b2, and b3 (which is only 2,149 bytes long). For each block, a hash value is
generated using the SHA-256 hash function. So for MyFile.mp4, those hashes will be denoted
as [h0, h1, h2, h3] in the same order as the blocks. This set of hash values, in order, represents
the file, and is referred to as the hashlist. Note that if you are given a block, you can compute its
hash by applying the SHA-256 hash function to the block. This also means that if you change
data in a block the hash value will change as a result. To update a file, you change a subset of
the bytes in the file, and recompute the hashlist. Depending on the modification, at least one,
but perhaps all, of the hash values in the hashlist will change
Files and filenames
Files in SurfStore are denoted by filenames, which are represented as strings. For example
“MyDog.jpg”, “WinterVacation.mp4”, and “Expenses.txt” are all examples of filenames.
SurfStore doesn’t have a concept of a directory or directory hierarchy–filenames are just strings.
For this reason, filenames can only be compared for equality or inequality, and there are no “cd”
or “mkdir” commands. Filenames are case sensitive, meaning that “Myfile.jpg” is different than
“myfile.jpg”. Filenames can contain spaces, but as described below, cannot contain commas ‘,’
and cannot be named index.txt.
The base directory
A command-line argument specifies a “base directory” for the client. This is the directory that is
going to be synchronized with your cloud-based service. Your client will upload files from this
base directory to the cloud, and download files (and changes to files) from the cloud into this
base directory. Your client should not modify any files outside of this base directory. Note in
particular–your client should not download files into the “current” directory, only the directory
specified by that command line argument.
Each file/filename is associated with a version, which is a monotonically increasing
non-negative integer. The version is incremented any time the file is created, modified, or
deleted. The purpose of the version is so that clients can detect when they have an out-of-date
view of the file hierarchy.
For example, imagine that Tahani wants to update a spreadsheet file that tracks conference
room reservations. Ideally, they would perform the following actions:
However, another client might be concurrently modifying this file as well. In reality, the order of
operations might be:
As you can see, Tahani overwrote the change that Jianyu made without realizing it. We can
solve this problem with file versions. Every time a file is modified, its version number is
incremented. SurfStore only records modifications to files if the version is exactly one larger
than the currently recorded version. Let’s see what would happen in the two-client case:
In the above example, both Tahani and Jianyu downloaded identical copies of confroom.txt (at
version 5). They then both started editing their local copies of the file. So there was a point
where Tahani had “her own” version 6 of the file (with her local changes), and Jianyu had “his
own” version 6 of the file (with his local changes). How do we know whose “version 6” is the real
The answer is that whoever syncs their changes to the cloud first wins. So in this example,
Jianyu was first to sync his changes to the cloud, which caused his modifications to the file to
become the official version 6 of the file. Later, when Tahani tries to upload her changes, she
realizes that Jianyu beat her to it, and so Jianyu’s changes to the file will overwrite her copy.
To delete a file, the MetadataStore service records a versioned “tombstone” update. This
update simply indicates that the file has been deleted. In this way, deletion events also require
version numbers, which prevents race conditions that can occur when one client deletes a file
concurrently with another client deleting that file. Note that this means that a deleted file must be
recreated before it can be read by a client again. If a client tries to delete a file that has already
been deleted, that is fine–just handle the version numbers of these tombstone updates
To represent a “tombstone” record, we will set the file’s hash list to a single hash value of “0” (a
string with one character which is the 0 character).
Your client will “sync” a local base_dir base directory with your SurfStore cloud service. When
you invoke your client, the sync operation will occur, and then the client will exit. As a result of
syncing, new files added to your base directory will be uploaded to the cloud, files that were
sync’d to the cloud from other clients will be downloaded to your base directory, and any files
which have “edit conflicts” will be resolved. Resolving conflicts will be described below.
A simple example, assuming that Tahani keeps her files in /tdata, and Jianyu keeps his files in
tahani $ ls /tdata
(Tahani’s base directory starts empty)
tahani $ cp ~/kitten.jpg /tdata
tahani $ ./run-client myserver.ucsd.edu:5001 /tdata 4096
(syncs kitten.jpg to the server hosted on myserver.ucsd.edu port 5001, using
/tdata as the base directory, with a block size of 4096 bytes)
jianyu$ ls /jdata
(Jianu’s base directory starts empty)
jianyu $ ./run-client myserver.ucsd.edu:5001 /jdata 4096
(kitten.jpg gets sync’d from the server hosted on myserver.ucsd.edu port
5001, using /jdata as the base directory, with a block size of 4096 bytes)
jianyu $ ls /jdata
Your client program will create and maintain an index.txt file in the base directory which holds
local, client-specific information that must be kept between invocations of the client. If that file
doesn’t exist, your client should create it. In particular, the index.txt contains a copy of the
server’s FileInfoMap accurate as of the last time that sync was called. The purpose of this index
file is to detect files that have changed, or been added to the base directory since the last time
that the client executed.
The format of the index.txt file should be one line per file, and each line should have the
filename, the version, and then the hash list. The filename is separated from the version
number with a comma, and the version number is separated from the hash list with a comma.
Entries in the hash list should be separated by spaces. For example:
File1.dat,3,h0 h1 h2 h3
Note that the name “index.txt” is special, and so our system does not allow you to sync regular
files with that name. In reality, a client like Dropbox stores metadata hashes in a hidden file
(e.g. .index) but we’re just going to use a regular file.
Storing blocks in the BlockStore service
The BlockStore server stores the contents of each block of data, indexed by its hash value. It
supports basic get and put operations. It does not need to support deleting blocks of data–we
just let unused blocks remain in the store. The BlockStore service only knows about blocks–it
doesn’t know anything about how blocks relate to files.
The service implements the following API:
● putblock(b): Stores block b in the key-value store, indexed by hash value h
● b = getblock(h): Retrieves a block indexed by hash value h
● hashlist out = hasblocks (hashlist in): Given a list of hashes “in”, returns a list containing
the subset of in that are stored in the key-value store
The Metadata server maintains the mapping of filenames to hashlists (and version numbers)
called a FileInfoMap. When we start to test your project, we will always start from a “clean slate”
in which there are no files in the system.
The service implements the following API:
● getfileinfomap(): Returns a mapping of the files stored in the SurfStore cloud service,
including the version, filename, and hashlist.
● updatefile(): Updates the FileInfo values associated with a file stored in the cloud. This
method replaces the hash list for the file with the provided hash list only if the new
version number is exactly one greater than the current version number. Otherwise, an
error is sent to the client telling them that the version they are trying to store is not right
(likely too old) as well as the current value of the file’s version on the server.
To create a file that has never existed, use the update_file() API call with a version number set
to 1. To create a file that was previously deleted, update the version number that is one larger
than the “tombstone” record.
Basic operating theory
When a client syncs its local base directory with the cloud, a number of things must be done to
properly complete the sync operation.
The client should first scan the base directory, and for each file, compute that file’s hash list.
The client should then consult the local index file and compare the results, to see whether (1)
there are now new files in the base directory that aren’t in the index file, or (2) files that are in
the index file, but have changed since the last time the client was executed (i.e., the hash list is
Next, the client should connect to the server and download an updated FileInfoMap. For the
purposes of this discussion, let’s call this the “remote index.”
The client should now compare the local index (and any changes to local files not reflected in
the local index) with the remote index. A few things might result.
First, it is possible that the remote index refers to a file not present in the local index or in the
base directory. In this case, the client should download the blocks associated with that file,
reconstitute that file in the base directory, and then add the updated FileInfo information to the
Next, it is possible that there are new files in the local base directory that aren’t in the local index
or in the remote index. The client should upload the blocks corresponding to this file to the
server, then update the server with the new FileInfo. If that update is successful, then the client
should update its local index. Note it is possible that while this operation is in progress, some
other client makes it to the server first, and creates the file first. In that case, the update_file()
operation will fail with a version error, and the client should handle this conflict as described in
the next section.
Can we assume that the files on the client wont change for the duration of sync?
● If Client 1 is executing a sync operation, you can assume that none of the files on Client
1’s computer are changing. However, it is possible that a different Client, say Client 2, is
interacting with the server. So two clients could by sync’ing at the same time.
The above discussion assumes that a file existed in the server that wasn’t locally present, or a
new file was created locally that wasn’t on the server. Both of these cases are pretty
straightforward (simply upload or download the file as appropriate). But what happens when
there is some kind of version mismatch, as described in the motivation at the top of this
specification? We describe what to do in this subsection.
Imagine that for a file like cat.jpg, the local index shows that file at version 3, and we compare
the hash list in the local index with the file contents, and confirm that there are no local
modifications to the file. We then look at the remote index, and see that the version on the
server is larger, for example 4. In this case, the client should download any needed blocks from
the server to bring cat.jpg up to version 4, then reconstitute cat.jpg to become version 4 of that
file, and finally the client should update its local index, bringing that entry to version 4. At this
point, the changes from the cloud have been merged into the local file.
Consider the opposite case: the client sees that its local index references cat.jpg with version 3.
The client compares the hash list in the local index to the file contents, and sees that there are
uncommitted local changes (the hashes differ). The client compares the local index to the
remote index, and sees that both indexes are at the same version (in this case, version 3). This
means that we need to sync our local changes to the cloud. The client can now update the
mapping on the server, and if that RPC call completes successfully, the client can update the
entry in the local index and is done (there is no need to modify the file’s contents in the base
directory in this case).
Finally, we must consider the case where there are local modifications to a file (so, for example,
our local index shows the file at version 3, but the file’s contents do not match the local index).
Further, we see that the version in the remote index is larger than our local index. What should
we do in this case? Well, we follow the rule that whoever syncs to the cloud first wins. Thus, we
must go with the version of the file already synced to the server. So we download any required
blocks and bring our local version of the file up to date with the cloud version.
For this project, you will use command line arguments to specify the server IP address, port
number, the directory to use as a “base directory”. Further, the block size is specified as a
command line option as well, and you should use that instead of a hard-coded number. Note
that while your system must support different-sized blocks (specified on the command line), the
size of the block will remain constant during any particular series of tests. So we might
run a set of tests with a block size of 4096, then clear everything and run a totally different set of
tests in a new environment with a block size of 1 megabyte (for example).
● Sanity checking: calling sync with an empty base directory and empty server doesn’t
result in an error, etc.
● New local files sync to the cloud
● Files on the server that aren’t on the client sync to the client
● Mixtures of new and missing files sync properly–the new files get uploaded to the cloud,
and the missing files get downloaded properly
● Updates: if the cloud has changes to a local file, those changes get downloaded
properly. If the local client has changes that aren’t on the cloud, those changes get
● Conflicts: Your client and server handle the case where multiple clients have modified a
file concurrently, and the above described rules are followed.
● Make sure that your program generates/reads/uses a local index.txt file. Make sure the
format of that file matches the above spec because our testing program might need to
read e.g. the version number of a particular file.
● Verify that your program works with binary files (images, video, etc).
● Make sure your program uses the block size specified in the command line argument.
Don’t hard-code a 4096 byte block size.
● When updating or creating a file, upload the necessary blocks to the block store first,
before updating the fileinfo map on the server. This way, there isn’t a race condition
where a client downloads the fileinfo map, then tries to download the blocks but they
haven’t been uploaded yet.
Submitting your work
Access GradeScope via Canvas so we can ensure that your grades sync properly.
The due date/time are listed on the course calendar/schedule.
FAQ / Questions / Updates
Can we change the starter code?
● Yes, so long as the various requirements in the specification are still met, but don’t
change the RPC interface itself or the data types passed through RPC
Will you use our client with our server?
● Yes, we will use your client with your server
As we execute a sync operation, if we notice that a file has changed and we update it, do we
start over and re-check every file again? Or just process the files we haven’t processed yet?
● Each time your client runs, it should “sync” the local files and any remote files. But it
should only process the files that haven’t been processed yet. For example, if you have
a base directory with files A, B, C, and D in it, and you read and process A, then read
and process B, if you notice that B needs to be updated (and you update it), then go on
to C and D. Don’t go back and re-check A again.
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