diff --git a/src/app.rs b/src/app.rs
index 75212a95e66..6d56a488cae 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -69,6 +69,11 @@ impl App {
             (_, Env::Test) => 1,
             _ => 30,
         };
+        let read_only_mode = dotenv::var("READ_ONLY_MODE").is_ok();
+        let connection_config = db::ConnectionConfig {
+            statement_timeout: db_connection_timeout,
+            read_only: read_only_mode,
+        };
 
         let thread_pool = Arc::new(ScheduledThreadPool::new(db_helper_threads));
 
@@ -76,7 +81,7 @@ impl App {
             .max_size(db_pool_size)
             .min_idle(db_min_idle)
             .connection_timeout(Duration::from_secs(db_connection_timeout))
-            .connection_customizer(Box::new(db::SetStatementTimeout(db_connection_timeout)))
+            .connection_customizer(Box::new(connection_config))
             .thread_pool(thread_pool);
 
         App {
diff --git a/src/db.rs b/src/db.rs
index 214266f48a9..831ea767696 100644
--- a/src/db.rs
+++ b/src/db.rs
@@ -100,15 +100,26 @@ impl<T: Request + ?Sized> RequestTransaction for T {
 }
 
 #[derive(Debug, Clone, Copy)]
-pub struct SetStatementTimeout(pub u64);
+pub struct ConnectionConfig {
+    pub statement_timeout: u64,
+    pub read_only: bool,
+}
 
-impl CustomizeConnection<PgConnection, r2d2::Error> for SetStatementTimeout {
+impl CustomizeConnection<PgConnection, r2d2::Error> for ConnectionConfig {
     fn on_acquire(&self, conn: &mut PgConnection) -> Result<(), r2d2::Error> {
         use diesel::sql_query;
 
-        sql_query(format!("SET statement_timeout = {}", self.0 * 1000))
-            .execute(conn)
-            .map_err(r2d2::Error::QueryError)?;
+        sql_query(format!(
+            "SET statement_timeout = {}",
+            self.statement_timeout * 1000
+        ))
+        .execute(conn)
+        .map_err(r2d2::Error::QueryError)?;
+        if self.read_only {
+            sql_query("SET default_transaction_read_only = 't'")
+                .execute(conn)
+                .map_err(r2d2::Error::QueryError)?;
+        }
         Ok(())
     }
 }
diff --git a/src/git.rs b/src/git.rs
index 00b598a07b3..07f454c85a9 100644
--- a/src/git.rs
+++ b/src/git.rs
@@ -12,7 +12,7 @@ use url::Url;
 use crate::background_jobs::Environment;
 use crate::models::{DependencyKind, Version};
 use crate::schema::versions;
-use crate::util::errors::{internal, std_error_no_send, CargoResult};
+use crate::util::errors::{std_error_no_send, CargoError, CargoResult};
 
 #[derive(Serialize, Deserialize, Debug)]
 pub struct Crate {
@@ -159,7 +159,9 @@ impl Job for AddCrate {
 }
 
 pub fn add_crate(conn: &PgConnection, krate: Crate) -> CargoResult<()> {
-    AddCrate { krate }.enqueue(conn).map_err(|e| internal(&e))
+    AddCrate { krate }
+        .enqueue(conn)
+        .map_err(|e| CargoError::from_std_error(e))
 }
 
 #[derive(Serialize, Deserialize)]
@@ -239,5 +241,5 @@ pub fn yank(conn: &PgConnection, krate: String, version: Version, yanked: bool)
         yanked,
     }
     .enqueue(conn)
-    .map_err(|e| internal(&e))
+    .map_err(|e| CargoError::from_std_error(e))
 }
diff --git a/src/middleware/current_user.rs b/src/middleware/current_user.rs
index 577d1ac2040..8e96214d3c8 100644
--- a/src/middleware/current_user.rs
+++ b/src/middleware/current_user.rs
@@ -45,7 +45,9 @@ impl Middleware for CurrentUser {
             // Otherwise, look for an `Authorization` header on the request
             // and try to find a user in the database with a matching API token
             let user = if let Some(headers) = req.headers().find("Authorization") {
-                User::find_by_api_token(&conn, headers[0]).ok()
+                User::find_by_api_token(&conn, headers[0])
+                    .optional()
+                    .map_err(|e| Box::new(e) as Box<dyn Error + Send>)?
             } else {
                 None
             };
diff --git a/src/middleware/run_pending_background_jobs.rs b/src/middleware/run_pending_background_jobs.rs
index ee120503db6..5023aee7a7a 100644
--- a/src/middleware/run_pending_background_jobs.rs
+++ b/src/middleware/run_pending_background_jobs.rs
@@ -13,6 +13,10 @@ impl Middleware for RunPendingBackgroundJobs {
         req: &mut dyn Request,
         res: Result<Response, Box<dyn Error + Send>>,
     ) -> Result<Response, Box<dyn Error + Send>> {
+        if response_is_error(&res) {
+            return res;
+        }
+
         let app = req.app();
         let connection_pool = app.diesel_database.clone();
         let repo = Repository::open(&app.config.index_location).expect("Could not clone index");
@@ -28,3 +32,10 @@ impl Middleware for RunPendingBackgroundJobs {
         res
     }
 }
+
+fn response_is_error(res: &Result<Response, Box<dyn Error + Send>>) -> bool {
+    match res {
+        Ok(res) => res.status.0 >= 400,
+        Err(_) => true,
+    }
+}
diff --git a/src/models/user.rs b/src/models/user.rs
index 5efc0a00d1f..f40dbded535 100644
--- a/src/models/user.rs
+++ b/src/models/user.rs
@@ -106,18 +106,26 @@ impl<'a> NewUser<'a> {
 
 impl User {
     /// Queries the database for a user with a certain `api_token` value.
-    pub fn find_by_api_token(conn: &PgConnection, token_: &str) -> CargoResult<User> {
+    pub fn find_by_api_token(conn: &PgConnection, token_: &str) -> QueryResult<User> {
         use crate::schema::api_tokens::dsl::{api_tokens, last_used_at, revoked, token, user_id};
-        use crate::schema::users::dsl::{id, users};
         use diesel::update;
+
         let tokens = api_tokens
             .filter(token.eq(token_))
             .filter(revoked.eq(false));
-        let user_id_ = update(tokens)
-            .set(last_used_at.eq(now.nullable()))
-            .returning(user_id)
-            .get_result::<i32>(conn)?;
-        Ok(users.filter(id.eq(user_id_)).get_result(conn)?)
+
+        // If the database is in read only mode, we can't update last_used_at.
+        // Try updating in a new transaction, if that fails, fall back to reading
+        let user_id_ = conn
+            .transaction(|| {
+                update(tokens)
+                    .set(last_used_at.eq(now.nullable()))
+                    .returning(user_id)
+                    .get_result::<i32>(conn)
+            })
+            .or_else(|_| tokens.select(user_id).first(conn))?;
+
+        users::table.find(user_id_).first(conn)
     }
 
     pub fn owning(krate: &Crate, conn: &PgConnection) -> CargoResult<Vec<Owner>> {
diff --git a/src/tests/all.rs b/src/tests/all.rs
index 7b184a29139..35dea7a5aff 100644
--- a/src/tests/all.rs
+++ b/src/tests/all.rs
@@ -68,6 +68,7 @@ mod git;
 mod keyword;
 mod krate;
 mod owners;
+mod read_only_mode;
 mod record;
 mod schema_details;
 mod server;
diff --git a/src/tests/read_only_mode.rs b/src/tests/read_only_mode.rs
new file mode 100644
index 00000000000..55e59608b35
--- /dev/null
+++ b/src/tests/read_only_mode.rs
@@ -0,0 +1,56 @@
+use crate::builders::CrateBuilder;
+use crate::{RequestHelper, TestApp};
+use diesel::prelude::*;
+
+#[test]
+fn can_hit_read_only_endpoints_in_read_only_mode() {
+    let (app, anon) = TestApp::init().empty();
+    app.db(set_read_only).unwrap();
+    anon.get::<()>("/api/v1/crates").assert_status(200);
+}
+
+#[test]
+fn cannot_hit_endpoint_which_writes_db_in_read_only_mode() {
+    let (app, _, user, token) = TestApp::init().with_token();
+    app.db(|conn| {
+        CrateBuilder::new("foo_yank_read_only", user.as_model().id)
+            .version("1.0.0")
+            .expect_build(conn);
+        set_read_only(conn).unwrap();
+    });
+    token
+        .delete::<()>("/api/v1/crates/foo_yank_read_only/1.0.0/yank")
+        .assert_status(503);
+}
+
+#[test]
+#[ignore] // Will be implicitly fixed by #1387, no need to special case here
+fn can_download_crate_in_read_only_mode() {
+    let (app, anon, user) = TestApp::with_proxy().with_user();
+
+    app.db(|conn| {
+        CrateBuilder::new("foo_download_read_only", user.as_model().id)
+            .version("1.0.0")
+            .expect_build(conn);
+        set_read_only(conn).unwrap();
+    });
+
+    anon.get::<()>("/api/v1/crates/foo_download_read_only/1.0.0/download")
+        .assert_status(302);
+
+    // We're in read only mode so the download should not have been counted
+    app.db(|conn| {
+        use cargo_registry::schema::version_downloads::dsl::*;
+        use diesel::dsl::sum;
+
+        let dl_count = version_downloads
+            .select(sum(downloads))
+            .get_result::<Option<i64>>(conn);
+        assert_eq!(Ok(None), dl_count);
+    })
+}
+
+fn set_read_only(conn: &PgConnection) -> QueryResult<()> {
+    diesel::sql_query("SET TRANSACTION READ ONLY").execute(conn)?;
+    Ok(())
+}
diff --git a/src/util/errors.rs b/src/util/errors.rs
index 85455639bd0..49454a1d6e9 100644
--- a/src/util/errors.rs
+++ b/src/util/errors.rs
@@ -49,6 +49,22 @@ impl dyn CargoError {
     pub fn is<T: Any>(&self) -> bool {
         self.get_type_id() == TypeId::of::<T>()
     }
+
+    pub fn from_std_error(err: Box<dyn Error + Send>) -> Box<dyn CargoError> {
+        Self::try_convert(&*err).unwrap_or_else(|| internal(&err))
+    }
+
+    fn try_convert(err: &(dyn Error + Send + 'static)) -> Option<Box<Self>> {
+        match err.downcast_ref() {
+            Some(DieselError::NotFound) => Some(Box::new(NotFound)),
+            Some(DieselError::DatabaseError(_, info))
+                if info.message().ends_with("read-only transaction") =>
+            {
+                Some(Box::new(ReadOnlyMode))
+            }
+            _ => None,
+        }
+    }
 }
 
 impl CargoError for Box<dyn CargoError> {
@@ -155,13 +171,9 @@ impl<E: Error + Send + 'static> CargoError for E {
     }
 }
 
-impl<E: Any + Error + Send + 'static> From<E> for Box<dyn CargoError> {
+impl<E: Error + Send + 'static> From<E> for Box<dyn CargoError> {
     fn from(err: E) -> Box<dyn CargoError> {
-        if let Some(DieselError::NotFound) = Any::downcast_ref::<DieselError>(&err) {
-            Box::new(NotFound)
-        } else {
-            Box::new(err)
-        }
+        CargoError::try_convert(&err).unwrap_or_else(|| Box::new(err))
     }
 }
 // =============================================================================
@@ -340,3 +352,34 @@ pub fn std_error(e: Box<dyn CargoError>) -> Box<dyn Error + Send> {
 pub fn std_error_no_send(e: Box<dyn CargoError>) -> Box<dyn Error> {
     Box::new(CargoErrToStdErr(e))
 }
+
+#[derive(Debug, Clone, Copy)]
+pub struct ReadOnlyMode;
+
+impl CargoError for ReadOnlyMode {
+    fn description(&self) -> &str {
+        "tried to write in read only mode"
+    }
+
+    fn response(&self) -> Option<Response> {
+        let mut response = json_response(&Bad {
+            errors: vec![StringError {
+                detail: "Crates.io is currently in read-only mode for maintenance. \
+                         Please try again later."
+                    .to_string(),
+            }],
+        });
+        response.status = (503, "Service Unavailable");
+        Some(response)
+    }
+
+    fn human(&self) -> bool {
+        true
+    }
+}
+
+impl fmt::Display for ReadOnlyMode {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "Tried to write in read only mode".fmt(f)
+    }
+}