Multipart Rar
Iraida
I have been writing iPhone applications for some time now, sending data to server, receiving data (via HTTP protocol), without thinking too much about it. Mostly I am theoretically familiar with process, but the part I am not so familiar is HTTP multipart request. I know its basic structure, but the core of it eludes me.
It seems that whenever I am sending something different than plain text (like photos, music), I have to use a multipart request. Can someone briefly explain to me why it is used and what are its advantages?
As the official specification says, "one or more different sets of data are combined in a single body". So when photos and music are handled as multipart messages as mentioned in the question, probably there is some plain text metadata associated as well, thus making the request containing different types of data (binary, text), which implies the usage of multipart.
After seeing the error of my ways, I made them both the same asset type, then validated, saved and tried it again and it worked (?). Why was I getting a multipart geometry error when they were different asset types but not when they were the same? Something seemed off still.
I attempted it again, but this time with fresh new lines. I snapped the two different asset types together by changing the asset type mid-edit such that it didn't change the snapping rules. When I went to merge, I got "Selection must be from the same layer" from the Merge tool. That's fair - except that they were both primaries and from the same layer...
Multipart upload allows you to upload a single object as a set of parts. Each part is a contiguous portion of the object's data. You can upload these object parts independently and in any order. If transmission of any part fails, you can retransmit that part without affecting other parts. After all parts of your object are uploaded, Amazon S3 assembles these parts and creates the object. In general, when your object size reaches 100 MB, you should consider using multipart uploads instead of uploading the object in a single operation.
If you're uploading large objects over a stable high-bandwidth network, use multipart upload to maximize the use of your available bandwidth by uploading object parts in parallel for multi-threaded performance.
If you're uploading over a spotty network, use multipart upload to increase resiliency to network errors by avoiding upload restarts. When using multipart upload, you need to retry uploading only the parts that are interrupted during the upload. You don't need to restart uploading your object from the beginning.
For more information about using the Amazon S3 Express One Zone storage class with directory buckets, see What is S3 Express One Zone? and Directory buckets. For more information about using multipart upload with S3 Express One Zone and directory buckets, see Using multipart uploads with directory buckets.
Multipart upload is a three-step process: You initiate the upload, you upload the object parts, and after you have uploaded all the parts, you complete the multipart upload. Upon receiving the complete multipart upload request, Amazon S3 constructs the object from the uploaded parts, and you can then access the object just as you would any other object in your bucket.
When you send a request to initiate a multipart upload, Amazon S3 returns a response with an upload ID, which is a unique identifier for your multipart upload. You must include this upload ID whenever you upload parts, list the parts, complete an upload, or stop an upload. If you want to provide any metadata describing the object being uploaded, you must provide it in the request to initiate multipart upload.
When you upload a part, Amazon S3 returns an entity tag (ETag) for the part as a header in the response. For each part upload, you must record the part number and the ETag value. You must include these values in the subsequent request to complete the multipart upload. Each part will have its own ETag at the time of upload. However, once the multipart upload is complete and all parts are consolidated, all the parts will be under one ETag as a checksum of checksums.
After you initiate a multipart upload and upload one or more parts, you must either complete or stop the multipart upload to stop getting charged for storage of the uploaded parts. Only after you either complete or stop a multipart upload will Amazon S3 free up the parts storage and stop charging you for the parts storage.
After stopping a multipart upload, you cannot upload any part using that upload ID again. If any part uploads were in-progress, they can still succeed or fail even after you stop the upload. To make sure you free all storage consumed by all parts, you must stop a multipart upload only after all part uploads have been completed.
When you complete a multipart upload, Amazon S3 creates an object by concatenating the parts in ascending order based on the part number. If any object metadata was provided in the initiate multipart upload request, Amazon S3 associates that metadata with the object. After a successful complete request, the parts no longer exist.
Your complete multipart upload request must include the upload ID and a list of both part numbers and corresponding ETag values. The Amazon S3 response includes an ETag that uniquely identifies the combined object data. This ETag is not necessarily an MD5 hash of the object data.
For this example, assume that you are generating a multipart upload for a 100 GB file. In this case, you would have the following API calls for the entire process. There would be a total of 1002 API calls.
You can list the parts of a specific multipart upload or all in-progress multipart uploads. The list parts operation returns the parts information that you have uploaded for a specific multipart upload. For each list parts request, Amazon S3 returns the parts information for the specified multipart upload, up to a maximum of 1,000 parts. If there are more than 1,000 parts in the multipart upload, you must send a series of list part requests to retrieve all the parts. Note that the returned list of parts doesn't include parts that haven't finished uploading. Using the list multipart uploads operation, you can obtain a list of multipart uploads that are in progress.
An in-progress multipart upload is an upload that you have initiated, but have not yet completed or stopped. Each request returns at most 1,000 multipart uploads. If there are more than 1,000 multipart uploads in progress, you must send additional requests to retrieve the remaining multipart uploads. Use the returned listing only for verification. Do not use the result of this listing when sending a complete multipart upload request. Instead, maintain your own list of the part numbers that you specified when uploading parts and the corresponding ETag values that Amazon S3 returns.
When you upload an object to Amazon S3, you can specify a checksum algorithm for Amazon S3 to use. Amazon S3 uses MD5 by default to verify data integrity; however, you can specify an additional checksum algorithm to use. When using MD5, Amazon S3 calculates the checksum of the entire multipart object after the upload is complete. This checksum is not a checksum of the entire object, but rather a checksum of the checksums for each individual part.
When you instruct Amazon S3 to use additional checksums, Amazon S3 calculates the checksum value for each part and stores the values. You can use the API or SDK to retrieve the checksum value for individual parts by using GetObject or HeadObject. If you want to retrieve the checksum values for individual parts of multipart uploads still in process, you can use ListParts.
If you are using a multipart upload with additional checksums, the multipart part numbers must use consecutive part numbers. When using additional checksums, if you try to complete a multipart upload request with nonconsecutive part numbers, Amazon S3 generates HTTP 500 Internal Server Error error.
In a distributed development environment, it is possible for your application to initiate several updates on the same object at the same time. Your application might initiate several multipart uploads using the same object key. For each of these uploads, your application can then upload parts and send a complete upload request to Amazon S3 to create the object. When the buckets have S3 Versioning enabled, completing a multipart upload always creates a new version. For buckets that don't have versioning enabled, it is possible that some other request received between the time when a multipart upload is initiated and when it is completed might take precedence.
It is possible for some other request received between the time you initiated a multipart upload and completed it to take precedence. For example, if another operation deletes a key after you initiate a multipart upload with that key, but before you complete it, the complete multipart upload response might indicate a successful object creation without you ever seeing the object.
After you initiate a multipart upload, Amazon S3 retains all the parts until you either complete or stop the upload. Throughout its lifetime, you are billed for all storage, bandwidth, and requests for this multipart upload and its associated parts.
These parts are charged according to the storage class specified when the parts were uploaded. An exception to this are parts uploaded to S3 Glacier Flexible Retrieval or S3 Glacier Deep Archive. In-progress multipart parts for a PUT to the S3 Glacier Flexible Retrieval storage class are billed as S3 Glacier Flexible Retrieval Staging Storage at S3 Standard storage rates until the upload completes. In addition, both CreateMultipartUpload and UploadPart are billed at S3 Standard rates. Only the CompleteMultipartUpload request is billed at the S3 Glacier Flexible Retrieval rate. Similarly, in-progress multipart parts for a PUT to the S3 Glacier Deep Archive storage class are billed as S3 Glacier Flexible Retrieval Staging Storage at S3 Standard storage rates until the upload completes, with only the CompleteMultipartUpload request charged at S3 Glacier Deep Archive rates.
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