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// Copyright 2020 Oxide Computer Company /*! * Detailed end-user documentation for pagination lives in the Dropshot top- * level block comment. Here we discuss some of the design choices. * * ## Background: patterns for pagination * * [In their own API design guidelines, Google describes an approach similar to * the one we use][1]. There are many ways to implement the page token with * many different tradeoffs. The one described in the Dropshot top-level block * comment has a lot of nice properties: * * * For APIs backed by a database of some kind, it's usually straightforward to * use an existing primary key or other unique, sortable field (or combination * of fields) as the token. * * * If the client scans all the way through the collection, they will see every * object that existed both before the scan and after the scan and was not * renamed during the scan. (This isn't true for schemes that use a simple * numeric offset as the token.) * * * There's no server-side state associated with the token, so it's no problem * if the server crashes between requests or if subsequent requests are * handled by a different instance. (This isn't true for schemes that store * the result set on the server.) * * * It's often straightforward to support a reversed-order scan as well -- this * may just be a matter of flipping the inequality used for a database query. * * * It's easy to support sorting by a single field, and with some care it's * possible to support queries on multiple different fields, even at the same * time. An API can support listing by any unique, sortable combination of * fields. For example, say our Projects have a modification time ("mtime") * as well. We could support listing projects alphabetically by name _or_ in * order of most recently modified. For the latter, since the modification * time is generally not unique, and the marker must be unique, we'd really be * listing by an ("mtime" descending, "name" ascending) tuple. * * The interfaces here are intended to support this sort of use case. For APIs * backed by traditional RDBMS databases, see [this post for background on * various ways to page through a large set of data][2]. (What we're describing * here leverages what this post calls "keyset pagination".) * * Another consideration in designing pagination is whether the token ought to * be explicit and meaningful to the user or just an opaque token (likely * encoded in some way). It can be convenient for developers to use APIs where * the token is explicitly intended to be one of the fields of the object (e.g., * so that you could list animals starting in the middle by just requesting * `?animal_name=moose`), but this puts constraints on the server because * clients may come to depend on specific fields being supported and sorted in a * certain way. Dropshot takes the approach of using an encoded token that * includes information about the whole scan (e.g., the sort order). This makes * it possible to identify cases that might otherwise result in confusing * behavior (e.g., a client lists projects in ascending order, but then asks for * the next page in descending order). The token also includes a version number * so that it can be evolved in the future. * * * ## Background: Why paginate HTTP APIs in the first place? * * Pagination helps ensure that the cost of a request in terms of resource * utilization remains O(1) -- that is, it can be bounded above by a constant * rather than scaling proportionally with any of the request parameters. This * simplifies utilization monitoring, capacity planning, and scale-out * activities for the service, since operators can think of the service in terms * of one unit that needs to be scaled up. (It's still a very complex process, * but it would be significantly harder if requests weren't O(1).) * * Similarly, pagination helps ensure that the time required for a request is * O(1) under normal conditions. This makes it easier to define expectations * for service latency and to monitor that latency to determine if those * expectations are violated. Generally, if latency increases, then the service * is unhealthy, and a crisp definition of "unhealthy" is important to operate a * service with high availability. If requests weren't O(1), an increase in * latency might just reflect a changing workload that's still performing within * expectations -- e.g., clients listing larger collections than they were * before, but still getting results promptly. That would make it much harder * to see when the service really is unhealthy. * * Finally, bounding requests to O(1) work is a critical mitigation for common * (if basic) denial-of-service (DoS) attacks because it requires that clients * consume resources commensurate with the server costs that they're imposing. * If a service exposes an API that does work proportional to some parameter, * then it's cheap to launch a DoS on the service by just invoking that API with * a large parameter. By contrast, if the client has to do work that scales * linearly with the work the server has to do, then the client's costs go up in * order to scale up the attack. * * Along these lines, connections and requests consume finite server resources * like open file descriptors and database connections. If a service is built * so that requests are all supposed to take about the same amount of time (or * at least that there's a constant upper bound), then it may be possible to use * a simple timeout scheme to cancel requests that are taking too long, as might * happen if a malicious client finds some way to cause requests to hang or take * a very long time. * * [1]: https://cloud.google.com/apis/design/design_patterns#list_pagination * [2]: https://www.citusdata.com/blog/2016/03/30/five-ways-to-paginate/ */ use crate::error::HttpError; use crate::from_map::from_map; use base64::URL_SAFE; use schemars::JsonSchema; use serde::de::DeserializeOwned; use serde::Deserialize; use serde::Deserializer; use serde::Serialize; use std::collections::BTreeMap; use std::fmt::Debug; use std::num::NonZeroU64; /** * A page of results from a paginated API * * This structure is intended for use both on the server side (to generate the * results page) and on the client side (to parse it). */ #[derive(Debug, Deserialize, JsonSchema, Serialize)] #[schemars(description = "A single page of results")] pub struct ResultsPage<ItemType> { /** token used to fetch the next page of results (if any) */ pub next_page: Option<String>, /** list of items on this page of results */ pub items: Vec<ItemType>, } impl<ItemType> ResultsPage<ItemType> { /** * Construct a new results page from the list of `items`. `page_selector` * is a function used to construct the page token that clients will provide * to fetch the next page of results. `scan_params` is provided to the * `page_selector` function, since the token may depend on the type of scan. */ pub fn new<F, ScanParams, PageSelector>( items: Vec<ItemType>, scan_params: &ScanParams, get_page_selector: F, ) -> Result<ResultsPage<ItemType>, HttpError> where F: Fn(&ItemType, &ScanParams) -> PageSelector, PageSelector: Serialize, { let next_page = items .last() .map(|last_item| { let selector = get_page_selector(last_item, scan_params); serialize_page_token(selector) }) .transpose()?; Ok(ResultsPage { next_page, items, }) } } /** * Querystring parameters provided by clients when scanning a paginated * collection * * To build an API endpoint that paginates results, you have your handler * function accept a `Query<PaginationParams<ScanParams, PageSelector>>` and * return a [`ResultsPage`]. You define your own `ScanParams` and * `PageSelector` types. * * `ScanParams` describes the set of querystring parameters that your endpoint * accepts for the _first_ request of the scan (typically: filters and sort * options). This must be deserializable from a querystring. * * `PageSelector` describes the information your endpoint needs for requests * after the first one. Typically this would include an id of some sort for the * last item on the previous page as well as any parameters related to filtering * or sorting so that your function can apply those, too. The entire * `PageSelector` will be serialized to an opaque string and included in the * [`ResultsPage`]. The client is expected to provide this string as the * `"page_token"` querystring parameter in the subsequent request. * `PageSelector` must implement both [`Deserialize`] and [`Serialize`]. * (Unlike `ScanParams`, `PageSelector` will not be deserialized directly from * the querystring.) * * There are several complete, documented examples in `dropshot/examples`. * * **NOTE:** Your choices of `ScanParams` and `PageSelector` determine the * querystring parameters accepted by your endpoint and the structure of the * page token, respectively. Both of these are part of your API's public * interface, though the page token won't appear in the OpenAPI spec. Be * careful when designing these structures to consider what you might want to * support in the future. */ #[derive(Debug, Deserialize, JsonSchema)] pub struct PaginationParams<ScanParams, PageSelector> where ScanParams: DeserializeOwned, PageSelector: DeserializeOwned + Serialize, { /** * Specifies whether this is the first request in a scan or a subsequent * request, as well as the parameters provided * * See [`WhichPage`] for details. Note that this field is flattened by * serde, so you have to look at the variants of [`WhichPage`] to see what * query parameters are actually processed here. */ #[serde(flatten, deserialize_with = "deserialize_whichpage")] pub page: WhichPage<ScanParams, PageSelector>, /** * Client-requested limit on page size (optional) * * Consumers should use * [`RequestContext`][crate::handler::RequestContext::page_limit()] * to access this value. */ #[schemars( description = "Maximum number of items returned by a single call" )] pub(crate) limit: Option<NonZeroU64>, } /* * Deserialize `WhichPage` for `PaginationParams`. We In REST APIs, callers * typically provide either the parameters to resume a scan (in our case, just * "page_token") or the parameters to begin a new one (which can be * any set of parameters that our consumer wants). There's generally no * separate field to indicate which case they're requesting. We deserialize into * a generic map first and then either interpret the page token or deserialize * the map into ScanParams. */ fn deserialize_whichpage<'de, D, ScanParams, PageSelector>( deserializer: D, ) -> Result<WhichPage<ScanParams, PageSelector>, D::Error> where D: Deserializer<'de>, ScanParams: DeserializeOwned, PageSelector: DeserializeOwned, { let raw_params = BTreeMap::<String, String>::deserialize(deserializer)?; match raw_params.get("page_token") { Some(page_token) => { let page_start = deserialize_page_token(&page_token) .map_err(serde::de::Error::custom)?; Ok(WhichPage::Next(page_start)) } None => { let scan_params = from_map(&raw_params).map_err(serde::de::Error::custom)?; Ok(WhichPage::First(scan_params)) } } } /** * Describes whether the client is beginning a new scan or resuming an existing * one * * In either case, this type provides access to consumer-defined parameters for * the particular type of request. See [`PaginationParams`] for more * information. */ #[derive(Debug)] pub enum WhichPage<ScanParams, PageSelector> { /** * Indicates that the client is beginning a new scan * * `ScanParams` are the consumer-defined parameters for beginning a new scan * (e.g., filters, sort options, etc.) */ First(ScanParams), /** * Indicates that the client is resuming a previous scan * * `PageSelector` are the consumer-defined parameters for resuming a * previous scan (e.g., any scan parameters, plus a marker to indicate the * last result seen by the client). */ Next(PageSelector), } /* * Generate the JsonSchema for WhichPage from SchemaWhichPage. */ impl<ScanParams, PageSelector> JsonSchema for WhichPage<ScanParams, PageSelector> where ScanParams: JsonSchema, { fn schema_name() -> String { unimplemented!(); } fn json_schema( gen: &mut schemars::gen::SchemaGenerator, ) -> schemars::schema::Schema { SchemaWhichPage::<ScanParams>::json_schema(gen) } } /** * `ScanParams` for use with `PaginationParams` when the API endpoint has no * scan parameters (i.e., it always iterates items in the collection in the same * way). */ #[derive(Debug, Deserialize, JsonSchema)] pub struct EmptyScanParams {} /** * The order in which the client wants to page through the requested collection */ #[derive(Copy, Clone, Debug, Deserialize, JsonSchema, PartialEq, Serialize)] #[serde(rename_all = "lowercase")] pub enum PaginationOrder { Ascending, Descending, } /* * Token and querystring serialization and deserialization * * Page tokens essentially take the consumer's PageSelector struct, add a * version number, serialize that as JSON, and base64-encode the result. This * token is returned in any response from a paginated API, and the client will * pass it back as a query parameter for subsequent pagination requests. This * approach allows us to rev the serialized form if needed (see * `PaginationVersion`) and add other metadata in a backwards-compatiable way. * It also emphasizes to clients that the token should be treated as opaque, * though it's obviously not resistant to tampering. */ /** * Maximum length of a page token once the consumer-provided type is serialized * and the result is base64-encoded * * We impose a maximum length primarily to prevent a client from making us parse * extremely large strings. We apply this limit when we create tokens to avoid * handing out a token that can't be used. * * Note that these tokens are passed in the HTTP request line (before the * headers), and many HTTP implementations impose a limit as low as 8KiB on the * size of the request line and headers together, so it's a good idea to keep * this as small as we can. */ const MAX_TOKEN_LENGTH: usize = 512; /** * Version for the pagination token serialization format * * This may seem like overkill, but it allows us to rev this in a future version * of Dropshot without breaking any ongoing scans when the change is deployed. * If we rev this, we might need to provide a way for clients to request at * runtime which version of token to generate so that if they do a rolling * upgrade of multiple instances, they can configure the instances to generate * v1 tokens until the rollout is complete, then switch on the new token * version. Obviously, it would be better to avoid revving this version if * possible! * * Note that consumers still need to consider compatibility if they change their * own `ScanParams` or `PageSelector` types. */ #[derive(Copy, Clone, Debug, Deserialize, JsonSchema, PartialEq, Serialize)] #[serde(rename_all = "lowercase")] enum PaginationVersion { V1, } /** * Parts of the pagination token that actually get serialized */ #[derive(Debug, Deserialize, Serialize)] struct SerializedToken<PageSelector> { v: PaginationVersion, page_start: PageSelector, } /** * Construct a serialized page token from a consumer's page selector */ fn serialize_page_token<PageSelector: Serialize>( page_start: PageSelector, ) -> Result<String, HttpError> { let token_bytes = { let serialized_token = SerializedToken { v: PaginationVersion::V1, page_start: page_start, }; let json_bytes = serde_json::to_vec(&serialized_token).map_err(|e| { HttpError::for_internal_error(format!( "failed to serialize token: {}", e )) })?; base64::encode_config(json_bytes, URL_SAFE) }; /* * TODO-robustness is there a way for us to know at compile-time that * this won't be a problem? What if we say that PageSelector has to be * Sized? That won't guarantee that this will work, but wouldn't that * mean that if it ever works, then it will always work? But would that * interface be a pain to use, given that variable-length strings are * very common in the token? */ if token_bytes.len() > MAX_TOKEN_LENGTH { return Err(HttpError::for_internal_error(format!( "serialized token is too large ({} bytes, max is {})", token_bytes.len(), MAX_TOKEN_LENGTH ))); } Ok(token_bytes) } /** * Deserialize a token from the given string into the consumer's page selector * type */ fn deserialize_page_token<PageSelector: DeserializeOwned>( token_str: &str, ) -> Result<PageSelector, String> { if token_str.len() > MAX_TOKEN_LENGTH { return Err(String::from( "failed to parse pagination token: too large", )); } let json_bytes = base64::decode_config(token_str.as_bytes(), URL_SAFE) .map_err(|e| format!("failed to parse pagination token: {}", e))?; /* * TODO-debugging: we don't want the user to have to know about the * internal structure of the token, so the error message here doesn't * say anything about that. However, it would be nice if we could * create an internal error message that included the serde_json error, * which would have more context for someone looking at the server logs * to figure out what happened with this request. Our own `HttpError` * supports this, but it seems like serde only preserves the to_string() * output of the error anyway. It's not clear how else we could * propagate this information out. */ let deserialized: SerializedToken<PageSelector> = serde_json::from_slice(&json_bytes).map_err(|_| { format!("failed to parse pagination token: corrupted token") })?; if deserialized.v != PaginationVersion::V1 { return Err(format!( "failed to parse pagination token: unsupported version: {:?}", deserialized.v, )); } Ok(deserialized.page_start) } /* * This is the on-the-wire protocol; we use this solely to generate the schema. */ #[derive(JsonSchema)] #[allow(dead_code)] #[serde(untagged)] enum SchemaWhichPage<ScanParams> { Next { page_token: String }, First(ScanParams), } #[cfg(test)] mod test { use super::deserialize_page_token; use super::serialize_page_token; use super::PaginationParams; use super::ResultsPage; use super::WhichPage; use serde::de::DeserializeOwned; use serde::Deserialize; use serde::Serialize; use std::{fmt::Debug, num::NonZeroU64}; #[test] fn test_page_token_serialization() { #[derive(Deserialize, Serialize)] struct MyToken { x: u16, } #[derive(Debug, Deserialize, Serialize)] struct MyOtherToken { x: u8, } /* * The most basic functionality is that if we serialize something and * then deserialize the result of that, we get back the original thing. */ let before = MyToken { x: 1025, }; let serialized = serialize_page_token(&before).unwrap(); let after: MyToken = deserialize_page_token(&serialized).unwrap(); assert_eq!(after.x, 1025); /* * We should also sanity-check that if we try to deserialize it as the * wrong type, that will fail. */ let error = deserialize_page_token::<MyOtherToken>(&serialized).unwrap_err(); assert!(error.contains("corrupted token")); /* * Try serializing the maximum possible size. (This was empirically * determined at the time of this writing.) */ #[derive(Debug, Deserialize, Serialize)] struct TokenWithStr { s: String, } let input = TokenWithStr { s: String::from_utf8(vec![b'e'; 352]).unwrap(), }; let serialized = serialize_page_token(&input).unwrap(); assert_eq!(serialized.len(), super::MAX_TOKEN_LENGTH); let output: TokenWithStr = deserialize_page_token(&serialized).unwrap(); assert_eq!(input.s, output.s); /* * Error cases make up the rest of this test. * * Start by attempting to serialize a token larger than the maximum * allowed size. */ let input = TokenWithStr { s: String::from_utf8(vec![b'e'; 353]).unwrap(), }; let error = serialize_page_token(&input).unwrap_err(); assert_eq!(error.status_code, http::StatusCode::INTERNAL_SERVER_ERROR); assert_eq!(error.external_message, "Internal Server Error"); assert!(error .internal_message .contains("serialized token is too large")); /* Non-base64 */ let error = deserialize_page_token::<TokenWithStr>("not base 64").unwrap_err(); assert!(error.contains("failed to parse")); /* Non-JSON */ let error = deserialize_page_token::<TokenWithStr>(&base64::encode("{")) .unwrap_err(); assert!(error.contains("corrupted token")); /* Wrong top-level JSON type */ let error = deserialize_page_token::<TokenWithStr>(&base64::encode("[]")) .unwrap_err(); assert!(error.contains("corrupted token")); /* Structure does not match our general Dropshot schema. */ let error = deserialize_page_token::<TokenWithStr>(&base64::encode("{}")) .unwrap_err(); assert!(error.contains("corrupted token")); /* Bad version */ let error = deserialize_page_token::<TokenWithStr>(&base64::encode( "{\"v\":11}", )) .unwrap_err(); assert!(error.contains("corrupted token")); } /* * It's worth testing parsing around PaginationParams and WhichPage because * is a little non-trivial, owing to the use of untagged enums (which rely * on the ordering of fields), some optional fields, an extra layer of * indirection using `TryFrom`, etc. * * This is also the primary place where we test things like non-positive * values of "limit" being rejected, so even though the implementation in * our code is trivial, this functions more like an integration or system * test for those parameters. */ #[test] fn test_pagparams_parsing() { #[derive(Debug, Deserialize, Serialize)] struct MyScanParams { the_field: String, only_good: Option<String>, how_many: u32, really: bool, } #[derive(Debug, Deserialize)] struct MyOptionalScanParams { the_field: Option<String>, only_good: Option<String>, how_many: Option<i32>, for_reals: Option<bool>, } #[derive(Debug, Serialize, Deserialize)] struct MyPageSelector { the_page: u8, } /* * "First page" cases */ fn parse_as_first_page<T: DeserializeOwned + Debug>( querystring: &str, ) -> (T, Option<NonZeroU64>) { let pagparams: PaginationParams<T, MyPageSelector> = serde_urlencoded::from_str(querystring).unwrap(); let limit = pagparams.limit; let scan_params = match pagparams.page { WhichPage::Next(..) => panic!("expected first page"), WhichPage::First(x) => x, }; (scan_params, limit) } /* basic case: optional boolean specified, limit unspecified */ let (scan, limit) = parse_as_first_page::<MyScanParams>( "the_field=name&only_good=true&how_many=42&really=false", ); assert_eq!(scan.the_field, "name".to_string()); assert_eq!(scan.only_good, Some("true".to_string())); assert_eq!(scan.how_many, 42); assert_eq!(scan.really, false); assert_eq!(limit, None); /* optional boolean specified but false, limit unspecified */ let (scan, limit) = parse_as_first_page::<MyScanParams>( "the_field=&only_good=false&how_many=42&really=false", ); assert_eq!(scan.the_field, "".to_string()); assert_eq!(scan.only_good, Some("false".to_string())); assert_eq!(scan.how_many, 42); assert_eq!(scan.really, false); assert_eq!(limit, None); /* optional boolean unspecified, limit is valid */ let (scan, limit) = parse_as_first_page::<MyScanParams>( "the_field=name&limit=3&how_many=42&really=false", ); assert_eq!(scan.the_field, "name".to_string()); assert_eq!(scan.only_good, None); assert_eq!(scan.how_many, 42); assert_eq!(scan.really, false); assert_eq!(limit.unwrap().get(), 3); /* empty query string when all parameters are optional */ let (scan, limit) = parse_as_first_page::<MyOptionalScanParams>(""); assert_eq!(scan.the_field, None); assert_eq!(scan.only_good, None); assert_eq!(limit, None); /* extra parameters are fine */ let (scan, limit) = parse_as_first_page::<MyOptionalScanParams>( "the_field=name&limit=17&boomtown=okc&how_many=42", ); assert_eq!(scan.the_field, Some("name".to_string())); assert_eq!(scan.only_good, None); assert_eq!(scan.how_many, Some(42)); assert_eq!(limit.unwrap().get(), 17); /* * Error cases, including errors parsing first page parameters. * * TODO-polish The actual error messages for the following cases are * pretty poor, so we don't test them here, but we should clean these * up. */ fn parse_as_error(querystring: &str) -> serde_urlencoded::de::Error { serde_urlencoded::from_str::< PaginationParams<MyScanParams, MyPageSelector>, >(querystring) .unwrap_err() } /* missing required field ("the_field") */ parse_as_error(""); /* invalid limit (number out of range) */ parse_as_error("the_field=name&limit=0"); parse_as_error("the_field=name&limit=-3"); /* invalid limit (not a number) */ parse_as_error("the_field=name&limit=abcd"); /* * Invalid page token (bad base64 length) * Other test cases for deserializing tokens are tested elsewhere. */ parse_as_error("page_token=q"); /* * "Next page" cases */ fn parse_as_next_page( querystring: &str, ) -> (MyPageSelector, Option<NonZeroU64>) { let pagparams: PaginationParams<MyScanParams, MyPageSelector> = serde_urlencoded::from_str(querystring).unwrap(); let limit = pagparams.limit; let page_selector = match pagparams.page { WhichPage::Next(x) => x, WhichPage::First(_) => panic!("expected next page"), }; (page_selector, limit) } /* basic case */ let token = serialize_page_token(&MyPageSelector { the_page: 123, }) .unwrap(); let (page_selector, limit) = parse_as_next_page(&format!("page_token={}", token)); assert_eq!(page_selector.the_page, 123); assert_eq!(limit, None); /* limit is also accepted */ let (page_selector, limit) = parse_as_next_page(&format!("page_token={}&limit=12", token)); assert_eq!(page_selector.the_page, 123); assert_eq!(limit.unwrap().get(), 12); /* * Having parameters appropriate to the scan params doesn't change the * way this is interpreted. */ let (page_selector, limit) = parse_as_next_page(&format!( "the_field=name&page_token={}&limit=3", token )); assert_eq!(page_selector.the_page, 123); assert_eq!(limit.unwrap().get(), 3); /* invalid limits (same as above) */ parse_as_error(&format!("page_token={}&limit=0", token)); parse_as_error(&format!("page_token={}&limit=-3", token)); /* * We ought not to promise much about what happens if the user's * ScanParams has a "page_token" field. In practice, ours always takes * precedence (and it's not clear how else this could work). */ #[derive(Debug, Deserialize)] struct SketchyScanParams { page_token: String, } let pagparams: PaginationParams<SketchyScanParams, MyPageSelector> = serde_urlencoded::from_str(&format!("page_token={}", token)) .unwrap(); assert_eq!(pagparams.limit, None); match &pagparams.page { WhichPage::First(..) => { panic!("expected NextPage even with page_token in ScanParams") } WhichPage::Next(p) => { assert_eq!(p.the_page, 123); } } } #[test] fn test_results_page() { /* * It would be a neat paginated fibonacci API if the page selector was * just the last two numbers! Dropshot doesn't support that and it's * not clear that's a practical use case anyway. */ let items = vec![1, 1, 2, 3, 5, 8, 13]; let dummy_scan_params = 21; #[derive(Debug, Deserialize, Serialize)] struct FibPageSelector { prev: usize, } let get_page = |item: &usize, scan_params: &usize| FibPageSelector { prev: *item + *scan_params, }; let results = ResultsPage::new(items.clone(), &dummy_scan_params, get_page) .unwrap(); assert_eq!(results.items, items); assert!(results.next_page.is_some()); let token = results.next_page.unwrap(); let deserialized: FibPageSelector = deserialize_page_token(&token).unwrap(); assert_eq!(deserialized.prev, 34); let results = ResultsPage::new(Vec::new(), &dummy_scan_params, get_page).unwrap(); assert_eq!(results.items.len(), 0); assert!(results.next_page.is_none()); } }