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With this setup the locking will only be effective when parallel requests for tiles of the same meta tile go to the same MapProxy instance. You should configure MapProxy to write all lock files on a local filesystem to prevent this. ETag is an HTTP response header that allows remote users to obtain sensitive information like inode number, child process ids, and multipart MIME boundary. Since file locking doesn’t work well on most network filesystems you are likely to get errors when MapProxy writes these files on network filesystems. ETags (entity tags) are a well-known point of vulnerability in Apache web server. The other processes will wait till the the first process releases the lock and will then use the new created tile. With locking, only the first process will get the lock and request the meta tile. Without locking MapProxy would request the meta tile for each request. We’re SSH'd to our server as the root user. Instructions will be similar for CentOS 6, and also for Unmanaged CentOS 7 servers, with the same Apache version.
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This would typically happen when two or more requests for missing tiles are processed in parallel by MapProxy and these tiles belong to the same meta tile. For this article, we will be using a Core-Managed CentOS 7 VPS, which comes with a clean installation of Apache 2.4. MapProxy uses file locks to prevent that multiple processes will request the same image twice from a source.
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You can easily run multiple MapProxy instances in parallel and use a load balancer to distribute requests across all instances, but there are a few things to consider when the instances share the same tile cache with NFS or other network filesystems.